explain pls
Mary Sue is a character that the author tries to build up as an 'ideal character', but ends up being faux-perfect, and is trying too hard. A Mary Sue character usually is the center of the attention because the author is trying to make the character important, and a lot of times the character ends up taking away from the experience.
The name Mary Sue comes from a star trek fanfic that had an author self-insert that was written to be the most perfect and nice and best character, but ended up just being annoying.
In other words, the term "Mary Sue" is generally slapped on a character who is important in the story, possesses unusual physical traits, and has an irrelevantly over-skilled or over-idealized nature.
The TVtropes article explains it in quite detail, contrary to wikipedia.
