I know it's more complex, just if you are trying to explain what the Fediverse is to someone who's older

"Remember Usenet?"

"Just like that, but anyone can run a server and start their own newsgroups"

Imagine you have a big board on your front lawn where people can come to write stuff and respond to others on the board. This board is an instance.

Your neighbor has their own board, which they have “federated” with yours. Messages from your board can show up on their board, and people there can write on those messages same as ones native to that board.

You can federate with them so their stuff shows on your board, or defederate if you don’t like the people there.

Anyone with the ability to make a board can have one federated with other boards to make a really big web of boards, but to a person looking at your lawn’s board it feels like one big one.

It’s a gargantuan group of forums that you can access through your home forum. Your home forum admins act like admins on your home forum but have no power elsewhere.

Or “It’s like Reddit, but with longer URLs and a slightly different user culture.”