@brizaa It's a slang term that comes from "simple" meaning they are stupid.
However I've been hearing people use simp to mean they like someone, but the person they like is not perfect. They might be saying that they like this person an unreasonable amount, more than you probably think they should. They are trying to avoid other people telling them that they shouldn't like that person by preemptively telling you that they know they are stupid for liking them.
A: I'm such a simp for Anakin Skywalker. B: Oh my god, you know he killed a bunch of children, right? A: Yeah, but he's still so hot and talented.