In American culture, 'Otaku' means 'One who is obsessed with Japanese culture, entertainment, etc.'
It literally is the honorific way to say 'House', and is used to mean 'Someone who stays in their house all the time'.
It's not usually a good thing to be called an Otaku, but others take pride in it.
Person 1: I love anime!
Person 2: You're such an Otaku!