It means they consider themselves genderless.
He may consider himself genderless, but he still has a sex, male or female. For someone to say they they are non-binary means that they are admitting they have mental problems.
When it comes down to it, this She/He/IT is a fruitcake that killed and maimed a bunch of other fruitcakes.
It means they’re mentally ill.
Nonbinary reject the idea that there are 2 genders.