Its a collection of self-hosted, usually open-source, federated social media platforms. They for the most part use Activity Pub as their underlining protocol so that platforms can communicate with each other to distribute the load of hosting social media instead of relying on a monolithic tech company to do all the hosting.
Im personally on Lemmy.World (federated Reddit clone) and its honestly a much healthier experience than Reddit but it lacks niche communities. Im hopeful for the Fediverse, but so far its really only populated by the tech-literate socially conscious crowd or extremely political fringes that are persona non grata elsewhere, with very little in between.