An instance is a specific computer a person uses .

Let’s say lem.ee is still up and you login there and have an account there but we both browse the rpgmemes on .ttrpg

Lem.ee is an instance, lemmy.world is an instance the .ttrpg computer is an instance. Like different email servers , these computers all talk with each other to make it seamless. All of the computers use the Lemmy software.

Now lem.ee has shut down, users can’t login there , but a new Lemmy instance could be created by someone and connect to the federated network.

But a university might run it’s own instance of Lemmy, giving students an account and have their own communities which aren’t publicly shared and they aren’t federated.

People can say there’s an issue with federation where people may leave comments on another computers community but they’re not visible to users browsing others. E.g. users on lemmy.workd might see it and .ttrpg but not anywhere else. They are meant to but an issue like slow communication is preventing it not permissions as such.

A fork is a different version of software. Open source software licences allw people to modify and re release the programs.

two developers might have different visions of what a software should look like , and if they want to split and make their own versions, that would be a fork in the development of the program.