It's kind of similar to nostr, although instead of a pair of keys, you get connect to the servers and create a one time link to send to other people to start a one on one conversation (or a group chat). I find it very easy and intuitive to use, it's taken me less than 5 minutes from installation to be participating on a group and maintaining private chats.

I like nost much, much, much better, though, but I quite like SimpleX, too.