An author wishes to describe the appearance of their P.O.V. protagonist in detail, but as everything is described from their point of view, how can this be done unless they see themselves? One solution is to set the scene with the heroine inspecting herself in a mirror.
The protagonist will typically be a woman, very attractive, described in purple prose, and a Mary Sue (although none of these features are required for the trope to apply). Mary Sue's Mirror will make its appearance within the first few pages.
This trope is specific to written fiction. It is common in amateur fiction, but much rarer in professional writing.