Your instance is the web domain that is hosting Lemmy. So, the instance you joined is lemmy.world. There are many instances, like lemmy.world, such as lemmy.zip, lemmy.today, lemmy.ca, and lemmy.ml (just to name a few). Additionally, there’s Piefed, and there are Piefed instances like Piefed.social (where I reside), piefed.world, etc. These instances all share content via “federation”.

When you’re logged into lemmy.world, you can use the built in search features to find communities (subreddits) you want to subscribe to, to see content posted there on your home feed.