When writing in 1st person POV, if your story's universe or your main character uses made up values of measurement or doesn't have a term for something like eyeliner or traffic lights, how do you clarify what's happening to the reader?
"I'll take 40 nargs of milk," Jon said, carefully counting out the gime needed to purchase it.
Stan's eyes went wide, and Jon waited for him to ask why he would need so much, but instead Stan pushed his hat back to look at the little cart he was hauling and said, "How you going to carry all that?"
"I'll manage somehow," Jon said. It turned out to be a bit more of a balancing act than he'd counted on, and one of the jugs fell out as he hit a hole in the road, but none spilled and he placed it back on.
You can get more descriptive than that. We don't need to know exactly how much a measure is, but you can give people a rough idea.
In the market they sold colorful silk fabric by the el. Will used to joke that an el in Drubinshire was longer than an el in Frisinki, because in Frisinki they used the king's personal part as the measure, and here they used the mayors'. She found herself missing Will's vulgar inaccuracies, even though she never could buy silk without thinking an el was overlarge if that's where the measure came from.
Still doesn't give us an inch for el measurement, but for the purposes of your story, you know that it's a measure of length, and it's longer than um...and this also tells us something about the characters.
You say that the character:
doesn't have a term for something like eyeliner or traffic lights
In this case, I take that to mean that they might be experiencing eyeliner or traffic lights for the first time (although eyeliner...that's really, really old or at least the idea of lining or darkening around your eyes is--sailors did it back in the day). So they either don't have a concept for it (traffic lights) or they do (eyeliner probably). If they do, it will remind them of something--a person with eyeliner or eyeshadow might remind them of sailors or pirates and that can be ever so much fun!!
Play with that--that they have things like it (history really does repeat itself and you would be surprised what your person might have seen in a different context) but that the context or meaning is totally different! Maybe only high priestesses darken round their eyes, or people with plague to mark them--or, or, or.
As to a never-before seen thing, like say a traffic light-- @FraEnrico's example is perfect, and I can't improve on it. It's situational though, depending on how observant the character is--they might not notice until after they get run over, and if they are walking might be more likely to notice the little walking man icon and the hand, along with the pedestrian flow.
We should have more details about the story you are writing, its setting and themes.
But generally speaking, I think about two scenarios:
1) the character talks about things that are unusual to their world, but known to the reader (i.e. a streetlight). In this case it is enough to describe it, and the reader can quickly grasp it.
Ex.: Walking down the street, Julius noticed how the vehicles moved according to the changing green and red light suspended above the crossing. It was like watching a religious rite, where a silent but authoritative priest would command his coloured orders to the obedient crowd of machines below. [Julius never saw a streetlight, we perfectly know what it's about]
2) the character talks about things that are common in their world, but unknown to the reader (i.e. a specific habit or lingo). In this case, the reader must accept the world as it is described by the characters. This is how the worldbuilding takes form.
Ex.: Julius jumped on the driftapan, which promptly moved on at the safe speed of 10 burgius per minute, as all the other transportation systems that were abiding by the nebularian laws. [i.e. we have no idea what a driftapan is and what is a burgius unit, but it's clear that we're talking about a vehicle and a measurement]
I would do what people actually do, and call it something descriptive. Here is an Online Etymology Dictionary that can tell you when and how various words in the English language originated and evolved, where they came from. It can be very helpful in giving you ideas for how to make up replacements.
You will find that most words are actually rather plain, shortened versions of metaphors or descriptions, or just trace to representative nouns: The word "line" comes from "linen" which comes from "linum", the Latin name of the flax plant from which linen fibers are extracted. "Flax" itself is a different name from a different language; and our word "flaxen" refers to the yellow blond color of the plant. Perhaps you can use a variation on "flax" for "line", or in your made-up language try to see this thing you must describe as if you have never seen one before, and decide what words might be used as shorthand.
This can be influenced by culture: Look up "traffic" and you will see it relates to trade or commerce, with the same root as "trans-", meaning "across", plus "fricare", for "friction". In fact a traffic light means a "signal to cross / not cross", both for cars and pedestrians, to reduce "crossing friction" (collisions).
Speaking of "signal" it comes from "sign" which meant "gesture".
Speaking of "Cross", it is from the latin word "crux" the name of the device upon which a crucifixion is accomplished.
A synonym for "crossing" is "intersection", your "traffic light" could be an "intersection manager".
The origin of words is obviously very idiosyncratic, based on other words and whatever concept happens to take hold first. But whatever the language, the procedure of finding a new name for an invention is seldom just an arbitrary made of set of sounds: It is descriptive. In "traffic light", "lights" were already often taken as a symbol: Remember in the midnight ride of Paul Revere, it was lanterns hung in a tower, "one if by land, two if by sea".
We are still not 100% settled on what to call electric lights: What we call "electric light" is probably an extension of very early distinctions between "candle light" and "day light" and "torch light", but we could have easily come to call them electric "lamps", "torches" or lanterns". In fact, electric flash lights are still called "torches", and "lamp" is now most commonly used to mean an electric lamp: It once referred to a wick/oil lamp, but now we almost always call that a "lantern".
It is better to root your made up words (or descriptions) in some history to make them more plausible, and also to reduce the burden of memorization on the reader.
Also, realize that for any ideas or objects we refer to frequently, we humans have a tendency to shorten for efficiency in speaking and writing. For example, "Crossing against the light" or "running the light", as phrases in isolation, are hard to make sense of, without the prior concept of a traffic light that manages an intersection to minimize the risk of collisions.
If possible, you want your made up words, coins, dress, behaviors, etc to be alternate versions of things we have: The reason we can translate most words in one language into another is because we humans have largely converged on a consensus of important concepts to be represented by language, and we differ only on the details of forming sounds and grammar.
So when you need words for "eyeliner" or "traffic light", treat them as a translation, and etymology (how the word came to me) can provide some fodder to help give you ideas of alternative ways a word might have plausibly evolved.
If your POV character just needs to describe (once) the function of a traffic light they have never seen before, exploring the etymology can help you get the idea of "painting the borders of the eyelids for contrast" or the idea of "managing a crossing to prevent collisions".