I think your usage of the word reveal is not fitting for what you ask about and may be at least in part a cause for your confusion.
When there is a secret or mystery that the reader wonders about (such as who the murderer is or whether the protagonist is human or not), this tension will be part of what keeps the reader reading and you have to be careful about when and how you allow him or her to learn the truth.
But if, as in this question, there is information that the reader didn't care about before, because it was irrelevant until now, then you aren't "revealing" the truth behind some mystery, you are simply providing the information that the reader needs to know to keep understanding what goes on.
In many, often older, novels, there is an exposition that introduces the characters and their circumstances to the reader. Think of the opening of The Hobbit: Tolkien describes what a hobbit is, how they live, and who Bilbo is in the first four paragraphs of the text:
In a hole in the ground there lived a hobbit. Not a nasty, dirty, wet hole, filled with the ends of worms and an oozy smell, nor yet a dry, bare, sandy hole with nothing in it to sit down on or to eat: it was a hobbit hole, and that means comfort.
It had a perfectly round door ... [description of Bag End, Bilbo's home]
This hobbit was a very well to do hobbit, and his name was Baggins. ... [description of Bilbo's character]
The mother of our particular hobbit … what is a hobbit? I suppose hobbits need some description nowadays ... [description of hobbits]
By some curious chance one morning ... [the actual story begins halfway down page 4 of the book]
In most contemporary novels, this exposition is missing and information such as age, appearance, drive, conflict, and past is provided as it becomes relevant.
Here is the beginning of The Hunger Games by Suzanne Collins:
When I wake up, the other side of the bed is cold. My fingers stretch out, seeking Prim's warmth but finding only the rough canvas cover of the matress. She must have had bad dreams and climbed in with out mother. Of course she did. This is the day of the reaping.
Collins doesn't explain who Prim and the viewpoint character are and how they live, before she begins the story. She simply throws us in medias res without forwarning and lets us figure things out as we follow her along.
She gives Prim's name, that we didn't know beforehand, when it becomes relevant. Katniss wakes, reaches out for someone, still half asleep, and notices that the person who she expected to be sleeping beside her is gone. She then comes more fully awake and thinks of that person more clearly. The concept of her sister appears in her mind, and that is when we are told the sister's name.
The narrative is restricted to the viewpoint character's external and internal perceptions. There are no explanations. At this point in the narrative, we don't yet know that Prim is the viewpoint character's sister, that her own name is Katniss, and what the reaping is. All we know is that a person named Prim had been sleeping beside the viewpoint character and that the reaping may have been causing Prim to have bad dreams. The explanations – who Prim is and what the reaping is – are presented to us as Katniss encounters them during the following day.
The opening of The Hunger Games takes this principle to an extreme, and there are mixed forms, where, for example, an explanation of who Prim is to the viewpoint character, might be given when she is first mentioned:
When I wake up, the other side of the bed is cold. My fingers stretch out, seeking Prim's warmth but finding only the rough canvas cover of the matress. At twelve years old, my sister Prim may be chosen to participate in the Hunger Games today. She must have had bad dreams and climbed in with out mother.
How you handle this is up to you and will depend on the style of your narrative. Is your narrator more omniscient and more of a guide to the reader? Then write an exposition and intersperse your narrative with excursions that explain background information to the reader. Is your viewpoint more that of how one character experiences the story? Then provide only the information that is relevant at each moment.
If you take this to extremes, your narrative may become hard to understand for the reader. This may be intriguing or irritating, depending on your target audience and writerly skill.
When I read William Faulkner's The Sound and the Fury I wasn't familiar with golf and his opening deeply mystified me. What do they hit?
Through the fence, between the curling flower spaces, I could see them hitting. They were coming toward where the flag was and I went along the fence. Luster was hunting in the grass by the flower tree. They took the flag out, and they were hitting. Then they put the flag back and they went to the table, and he hit and the other hit. Then they went on, and I went along the fence. Luster came away from the flower tree and we went along the fence and they stopped and we stopped and I looked through the fence while Luster was hunting in the grass.
If your readers are familiar with golf, though, that passage will be clearly transparent to them.
So, figure out who you are writing for, how much you want to challenge or how much you want to guide them, and provide information accordingly. If you are unsure, looking at how other books currently on the market in your genre do this may be helpful.
