Non-Binary is an umbrella term for everyone who doesn’t identify as either male or female. That coud be ‘None’ as you said which would be agender. But also everything in between. Like Demigender, where one identifies somewhat with one gender but not fully, Bigender, where one identifies as having two genders (which do not have to be male and female). And there are a lot more. But it’s not really important to know a lot of them as these are more labels for people to describe how they’re feeling, or how they’re expieriencing gender. If you respectfully ask a person who identifies as Non-Binary they’ll probably be happy to explain it a bit to you if they feel like doing it in general.