Basically, the Fediverse are lots of different sites that all use the same language (protocol), and some are able to talk to each other. So a Mastodon site (instance) like mastodon.world

can talk to another Mastodon instance (like mastodon.art, or techhub.social)
can talk to some other twitter-like platforms like Pleroma and Akkoma
can talk to instances of some other platforms including Lemmy and Mbin (reddit-like), Pixelfed (Instagram-like), Friendica (Facebook-like) and more
I'm not sure but I think you can like, comment and subscribe on PeerTube instances

I haven't kept up-to-date with what's possible and what isn't working yet, so I might have missed something.