It means the client won't even *try* to let you be a Moderator or an Admin. If you replace None with either of those, you do get a bunch of new buttons, but none of them will work since the server will know that you're not actually elevated. It's just a client-side setting.
I like to client-elevate myself to Admin because that way I can see the score of every save. Note that the score is not supposed to be hidden at all, everybody can see the score of every save on the website. It's just missing from the official client on elevation level None. Also note that jacob1's mod has it enabled by default.
Kinda like a "user number". (e.g. Simon is de first tpt registered account, so his User ID is 1)