The Concept of "the Fediverse" is an open network of services all speaking similar dialects of ActivityPub or another do-acratically designed web standards. Mastodon, Pleroma, Peertube, PixelFed all use this network protocol to communicate actor actions between nodes. It's similar to the IndieWeb in a lot of ways, but less focused on HTML and Semantic Markup but built around APIs and JSON.
I am @rrix@notes.whatthefuck.computer on the Fediverse.
NEXT How to participate in the fediverse
For now read Maya Kate's introduction to Mastodon: //maya.land/mastodon/