There’s a lot of technical answers being proposed but you really need to keep in mind your target audience. Do they actually care how it works at all, or are they less technically minded.

Take the email comparison, it makes an excellent example for people who know how email works but most people don’t know so it’s not helpful as a comparison to them.

For the layman it’s probably best to stick to a simple description, such as: The Fediverse is community organized social media, it works the same as Reddit/Twitter but it’s not owned by any company. There are a few extra steps when you first sign up, but it’s well worth it for the extra control you get.