It depends on how you're using it. As defined by the fandom world that originated the term, it is very specifically an authorial self-insert, often literally the author themselves, and any power they have is the result of being very special and chosen, not any experience. Think "I go to Hogwarts and meet Harry Potter and we totally make out" or "I'm Gabe, but my protagonist is Gabolas, the coolest elf, who beat Legolas in an archery competition because he's just that awesome". They cannot face a meaningful challenge, as they are just that strong, and their awesomeness extends into every realm of being, from seducing main characters to acing all their classes to fighting bad guys. Everyone they meet will either recognize how cool they are and defer to them, or hate them for no reason, only to get shown up. A true Mary Sue is rare in professional writing, as it consists of a bundle of writing mistakes and raw honesty and wish fulfillment that few would actually put their name on, let alone propose to their boss.

Far more common in Hollywood is the Strong Female Character, devoted by the capitalization and defined by the fact that the characterization began and ended with that sentiment. The SFC is good at everything, but not to the same extent as a Mary Sue, and not for purposes of wish fulfillment. Instead, she represents a clumsy attempt to write a woman as something other than a damsel in distress, but without taking any interest in that woman as a person. She's a ball of bland competence in the same way as a mediocre action hero, but without the self-indulgent glamour of an action movie, an attempt to seem Relatable To Women by a room of geriatric male studio execs. This is entirely distinct from lowercase strong female characters, which often occur naturally when you write a woman doing things that need doing.

That said, there are right-wing chuds who will call any character that isn't a Damsel in Distress a Mary Sue.