This is a Linux machine on the internet where you can get a shell account.
The files you put in ~/html are served by a webserver to tilde.fun/~yourusername.
You can apply for an account by connecting as the user tilde (no password) with ssh. messaging an admin (see below) but don't always pick the first one.
Any questions? Maybe they're answered in the FAQs.
(admins in orange)