It’s really a set of beliefs, usually centered around the idea that an individual can opt out of the legal system in favor of an alternative system. At a basic level, it’s generally someone who thinks their consent to be bound by a law determines if the law applies to them.
If you can change your “gender” just by declaring it, why not pretend you are under no laws and you can do anything you want and declare that you are your own country, or you are emperor of the Andromeda Galaxy Empire, or whatever. The left tells us you can be anything (except you can’t be a white, straight Christian).