People called Rey a Mary Sue because of the many hats she wore, like being a pilot/mechanic who can somehow fix the Millennium Falcon or defeat Kylo Ren in a lightsaber duel, despite never being trained.
For those who don’t know, Mary Sue is a term given to characters in fiction who “save the day through unrealistic abilities.”
Though the term can be used for both genders, it’s most often attached to female characters, which has led some to claim it’s a sexist term meant to keep down capable characters.
Essentially, it’s a character who can inexplicably solve any problem because they have to.