A Mary Sue (and the male equivalent, Gary Stu or Marty Stu) is a term for a fictional character, often in fanfiction but now in all genres, who is overly idealized and seemingly perfect.
Key Characteristics of a Mary Sue:
Unrealistic Perfection: They are exceptionally skilled, intelligent, powerful, and beautiful without having realistically earned it. They might be the best fighter, the smartest mage, and the most beautiful person in the room, all at once.
Lack of Flaws: Any flaws they have are either superficial ("clumsy") or are actually endearing qualities in disguise ("too trusting" or "works too hard").
Instant Likability: Every "good" character in the story loves them immediately and unconditionally.
Central to the Plot: The entire universe seems to revolve around them. Stories and conflicts bend to suit them.
Bends the World's Rules: They can solve problems that have stumped experts for years with ease, or they possess unique abilities that no one else has, breaking the established rules of the story's world.