In literature, a Mary Sue is an unbalanced character that is perfect in every way. They have no flaws, no weaknesses, no enemies that pose a true challenge, and always win, get everything they want, are usually wealthy, and are overly glorified at the expense of others.
In tagahanga fiction, Mary Sues will usually be partnered with a canonical character (one actually from the show) in defiance of the show's storyline, the pag-ibig interest picking the Mary Sue over the real story's partner. (As an example, a Mary Sue would win Jeremy from Candace in "Phinease and Ferb", or Heloise from Jimmy Two Shoes in "Jimmy Two Shoes".)
Mary Sues are universally despised for their lack of creativity, balance, and flaws. It is an immense insult to call an author's character a Mary Sue, so don't throw it lightly.