It can mean many things as it's both an umbrella-term for a number of identities alongside being one that many people choose to identify with specifically. Fundamentally it means that you don't identify as strictly a man or a woman - that it isn't a binary check mark in one of those boxes to you.
Naturally that's a very surface-level view as once you break outside of those two there's an entire spectrum between, but hopefully it starts to give a bit of an idea.