It's just another instance, but one that supports microblogging and some slightly different features. The fediverse is made out of instances.

Instances mainly affect where your data is physically stored (every instance has their own server) and what content moderators of your instance have decided to block (for example, they could decide to block content from a neo-nazi instance). You would still have access to all content from pretty much all other instances.

Other examples of Lemmy instances are beehaw.org, lemmy.blahaj.zone, lemmy.ml, sh.itjust.works, etc. You have a more exhaustive list at https://join-lemmy.org/instances