in the first 3 games same story you are a security guard and murderous animatronics are trying kill you
in 4 you are...... someone who is having "nightmares" of the fnaf 1 animatronics and you die somehow if they catch you (i say)
fnaf 5 i can't explain it is weird. fnaf 6 you are the dude from 1 and 3 who made your own pizzaria and have to not get sued and get that sweet, sweet cold hard cash.