Where a mirror is used as a narrative device to describe the main character.
Rolling Updates, probably Needs a Better Description, Needs More Examples, Do We Have This One?? Say you're writing a story from a certain person's POV, and you want to describe the main character without being awkward. Since the character's eyes are attached to their heads, they can hardly describe themselves, but there's a way around that - have the character look at themselves in a mirror, and describe what they see. This is usually slipped into a Morning Routine sequence, or when the character catches themselves in the mirror abroad. As well as being used for descriptive purposes, it can effectively be used to reflect character - suppose this person hates her appearance and usually avoids mirrors, or else closes in to focus on the flaws (pimples, wrinkles, whatever); have her get a good look at herself (say, in a mirror at someone else's house) where she can't spend time obsessing over her flaws, or are unable to close the distance due to a crowd (so that she's forced to stand back and get a view she doesn't often see). It could also be used to show their vanity - reflection on how gorgeous they look, or how they were sure no-one has as stunning [body feature]. But be warned, dear Writer - this trope is looked down on in some circles because of the amount of amateur fiction that use it poorly, either through lazyness - instead of feeding the audience a description piecemeal - or much more description than in the rest of the story - as then it's usually just another way to tell us how awesome your character is), as well as a percieved overuse of the trope (a popularity probably earned for much of the same reasons). That's not to say it can't be done well, just that it's easy to get wrong. Often uses the syntax "[Character] looked in the mirror, and a [description of character] looked back."
Harry ran his fingers over the scar again. It was still painful. He turned on the lamp beside him, scrambled out of bed, crossed the room, opened his wardrobe, and peered into the mirror on the inside of the door. A skinny boy of fourteen looked back at him, his bright green eyes puzzled under his untidy black hair. He examined the lightning-bolt scar of his reflection more closely. It looked normal, but it was still stinging.
She looked into the cruelly bright mirror, cruel because being so often used to check a disguising makeup, it revealed rather than softened every defect of complexion and feature. There was the real outward Ziantha. And with this hour and her great fatigue, that sight was a blow to any vanity.
She was very thin and her skin was pallid. Her hair, from the warm steam of her bath, curled tightly to her head, no lock of it longer than one of her fingers. In color it was silver fair, though in daylight it would show a little darker. Her eyes were gray, so pale as to seem silver too. The mouth below was large, her lips with little curve, but a clear red. As for the rest -- she scowled at the true Ziantha and shrugged on her night robe.