I know what a Karen is.
What is a Mary Sue?
Basically a self-inserted character the the writer projects into the story as a means of personal biases. Character has plot amour and basically operating in god mode. A very lazy way to write a character.
A Mary Sue is a person who is a woman character who is absolutely great at everything without any training or struggle to master a skill.
The girl heroine in the last batch of crappy "star wars" movies is an example of a Mary Sue.
Mary Poppins "Practically perfect in every way" would be a more benign example of a "Mary Sue", but from an era when her behavior wasn't intended to be a political message.
Mary Sues can beat up men twice their size, are instantly perfect at any sort of fighting or activity that requires mental capacity. They are super geniuses, and always win with little to no effort.
That's the gist of it.