The Fediverse is a network of thousands of independent social media servers of many different kinds.
The most popular kind is Mastodon, but it wasn't the first and isn't the only one. Some of the accounts you interact with are on other server types, e.g. @Iancylkowski is on Pixelfed and @theatticdwellers is on PeerTube. There are many Fediverse server types:
Whichever type you're signed up on, they connect together to form a social network called the Fediverse.
@feditips kind of want to try calckey but the lack of mobile apps is odd
It's very new compared to Mastodon, there hasn't been time for them to develop yet. Mastodon has been going since 2016.