*Correct me, anyone who sees this*

I think simp means when you're like, crushing on someone(particularly), that it's too overwhelming. Fictional or not, you care about them so much. You say: ''He's/She's mine!'' "Aaaa, he's/she's so cute! Jcskxksx I can't resist-''

Kinda like that,
I'm Filipino. I don't explain good in English.