Fediverse
How I explain
In my opinion, Fediverse is "Federated Universe," which means interconnected servers that use the ActivityPub protocol to exchange and distribute information to each other, such as social media posts (Mastodon, GotoSocial, Lemmy, and Threads), blogs (WriteFreely, Plume, and WordPress with the ActivityPub plugin).
Those software are usually free, libre, and open source, but some also use subscription-based marketing, such as managed hosting of the mentioned software, write.as, and micro.blog. Have fun experimenting with those software. My shortest Mastodon instance was ≤ 3 days lol.
So far, I know the Fediverse exists because those people with a huge amount of money destroyed that very popular social app by buying and owning it, lol. Many servers across the Fediverse mean that no single company can overtake single server.