Mastodon is part of the Fediverse.
The Fediverse is a collection of alternative social networks which are connected together through a common technical standard called ActivityPub.
People on the Fediverse can follow, comment, like or message each other seamlessly.
For example, the account @craftykat is on a video platform called PeerTube, but you can follow them, watch the videos, give them a like and comment on them without leaving Mastodon. Try it yourself!
Another example you can try interacting with is @Picturavis, which is on a photo platform called PixelFed.
When you use Mastodon, a lot of the accounts you talk to are actually not on Mastodon!
It can take a bit of time to get your head round this, because centralised social networks like Twitter or Facebook deliberately stifle interoperability.
The Fedi is different, no one owns it so there's no restriction on what people do with it. The ordinary user is all that matters on the Fedi 💪