What does it mean to be a fangirl/boy?
It literally just means you are big fan of something. Like you are a fangirl (assuming by the belle picture) of Disney. Or u could call urself a Disney fangirl. Pretty simple.
You support-like being a fan of someone.
You support what you love. I'm a juggalo fanboy.
it means to be completely obsessed with a certain show/book/movie series or anything in particular really
It just means you have a preference towards or you are partial towards one particular thing, person, or idea. Guess what I am a fanboy of? Don't look at my profile picture though, that's cheating.
it means to be an overly-obsessed fan of somebody or something.