How do you explain the Fediverse to someone who has never heard of it?

Simply put, think more like celphones and e-mail, and less like facebook. You can register an e-mail with google, yahoo, your ISP, your school, your job, or all of the above at the same time, and no matter which one you are using, you can still send a message to your mother for some god forsaken reason is still using an AOL e-mail address. If your e-mail provider is a jerk, throws too many ads, tracks you too much or simply hasn't added the features you want. you can just switch providers, and YOU can switch providers alone... and still e-mail all your old friends without having to beg them to change as well.