In my story, there is a group of four characters that are called upon a classic macguffin hunt. The characters know that the macguffin is important, but they do not know all the details as why it is important. I want to provide that info to my readers, but while writing in third person limited, I'm not sure how.
I think a generic answer would be better to help others, but read on for my particular case.
To draw a parallel to something in the real world, consider Tabasco brand sauce. Now, almost anyone can cultivate peppers almost anywhere and use them to make a hot sauce. But only if those peppers are that one particular genetic strain, and only if they are grown on Avery Island in the Gulf of Mexico off the coast of Louisiana, can the sauce made from those peppers carry the name Tabasco. There is something about the soil or the weather or whatever, that gives those peppers their unique blend of heat and flavor that no other hot sauce can replicate.
In my fictional world, this certain vegetable can also be grown almost anywhere by almost anyone. It can be used to add a pleasant flavor to tea, and also added during fermentation to make a nice liquour. But if you use "special" vegetable, that tea can treat a disease, and when special vegetable is used in the cool quaff, it adds an extra kick the regular version just can't replicate.
Because I am the author, I know that this is because special vegetable grows on a particular cluster of islands. These islands are of volcanic origin, and located within a equatorial zone. An ocean current brings a stable climate, regular rains, and steady supply of nutrients. A unique crab lives there, and its burrowing keeps the soil turned over just so. Finally, sea birds in the area go out and eat a unique blend of fish during the day, coming back at night to (um, er) fertilize the special vegetables.
The characters know about the medicinal tea and the potent potable, and they know about the existence of these islands. They do not know any of the rest. I believe it help my readers understand the magnitude of the quest if they knew the details. I can't write a dialog where one character explains to another, nor do I want to break in with a sudden paragraph of omniscient narrator, then jump back to a character's point of view. I can't find any way to do this that doesn't seem really jarring, unrealistic, nor break the flow of the story.