3. Nodes/instances/servers form the fediverse. Its a bit like like neighborhoods. You live somewhere, but you can also commute elsewhere. So you can communicate with people on another node. Your full username is @ username @ server.name (without spaces)
4. Different servers can have different features. (e.g. different length of character by post) different file size limits. They might also have different community rules. Take a moment to read your Server.name/about/more page to learn more.