It's when (usually early on) the character stares into a mirror as a narrative shortcut so the author can go down the physical description checklist for that character. Usually it's the second stop after someone's told "people don't really itemize their own physical description" in a critique, and then the author has them stare into a mirror, so they can have an excuse to itemize the character's physical description.

(The third step being: the author figures out that physical description of your protagonist in this blazon fashion is not, actually, required.)

Like many things that everybody does at an early point in their learning process, it's very, very hard to do meaningfully or well.