What can I do with Usenet?

You can enjoy threaded discussion in unmoderated groups without worrying about viewpoint censorship.

You can run your own NNTP peer and create your own newsgroups for your own community.

You can use a threaded newsreader application (like Thunderbird or Seamonkey or Claws Mail) which is much more efficient and organized than web forums.

... but I’ve got no idea what Usenet is.

You can start exploring Usenet here: https://news.novabbs.org