I'm probably not the best person to explain this, but it's when you believe you were a character in a past life mostly (that's the really simplest explanation I know it sucks ass). The concept is more like a reincarnation one and such, there isn't really much more tbh. People claim that is bad, in my perspective, because some people use problematic characters or even irl people (the irl people part is.. yikes to say the least) there is a carrd that can explain better then my ass so https://fkin.carrd.co/ (in my opinion fictionkin is a really "I don't give a fuck" situation, like if the person believes he's a incarnation then leave it alone they aren't doing anything harmful 😭)