an instance can be thought of like a reigonal server for a game, but for a community interest instead. dbzer0 is more on the fringes partaking more actively in piracy and AI shit, as well as other shit like anarchy and personal liberty/freedoms at a more broad scale.
Sometimes they’re regionally specific, like the midwest instance, other times they’re global like the .world instance.
you do have instance specific communities, and users obviously, but it’s also open to the broader “fediverse” as well. The only technicality is that i’m tied to dbzer0 since that’s where my acc sits, though i can still poke around outside of it.