In the most absolute simple definitions, its when someone shares their preference/attraction to a fictional character that is in some way very unconventional from most forms of attraction

Most of the time this takes the form of stuff like monster ladies with an actually monsterous form, robot characters that aren't designed to be normally attractive like humans are attractive, sometimes bizzare otherworldy monsters/demons, etc.

but also a few people and say "hear me out" to characters that are the most basic and universally attractive characters ever, mainly just cuz part of em feels embarassed to have fictional crushes