the hyper-abridged version:

Each "instance" (beehaw, lemmy.ml, lemmygrad, etc) is like its own small self-enclosed Reddit. "Federation" is the process that allows the instances to talk to each other - like for example, users registered at the beehaw instance can go and comment on posts from the lemmy.ml instance. This allows each instance to keep its own culture while granting the breadth of community of having every user able to interact with every other user. Some communities "defederate", which basically means an instance decides that for whatever reason they don't want their users interacting with users from another instance. So the network isn't fully connected, but instead forms a distributed network. Totally down to answer more quetsions despite my account only being like an hour old lol