So you're reading a book, watching a movie, or watching a tv show. You notice that there's this one character who is extremely beautiful, is smarter that most real life geniuses, everyone loves them, has few or no flaws, and otherwise displays many of the characteristics of a Sue or one of the Sue subtypes. Ok, they may not be as bad as the description above, but they still meet all Mary Sue Qualifications.
Then why is a large portion of the fandom not screaming for their blood?
Because, fellow troper, this character is a Lovable Sue.
This character has all the traits nescessary to make them a Sue, but for some reason, a vast majority of the fans for some reason like them
, rather than crying out for the blood of the character or the creator thereof like others would do.
See also Mary Sue
, and her many subtropes.