I've been explaining it like email. There's no email webpage you go to to create an account. It's just a protocall a bunch of people have agreed to use, so you go to one of them and you create an address. I also think your username in the fediverse should be called an address too, but I don't think that'll catch on. It makes it a lot easier to explain, because everyone can use email, even the most tech illiterate people.