lemmy.world and lemmy.ml are different “instances” of Lemmy. Anyone can host one, and every Lemmy instance can talk to other Lemmy instances, and similarly any other software using ActivityPub. It’s part of the “Fediverse”, or the federated universe.
In the long term, it won’t matter unless lemmy.ml starts defederating from other instances or other instances start defederating from it. “Defederating” is a term for when the admins of an instance choose not to talk to another specific instance.
Cool, I’m gaming on Linux :)