Lemmy.world. Which is ONE example of a Lemmy instance. Lemmy instances don’t even need to have Lemmy in the name.
Lemmy is a system that allows anyone to create what is essentially their own Reddit. Each of those are called instances. Lemmy.world is one of those, Lemmy.ml, is another, Beehaw is a third. Each of those Lemmy instances are run by different people for different reasons. Each of them have their own communities. A community is like a subreddit. The post you commented on (“PSA: Lemmy.ml is not Lemmy”) was posted to the “Fediverse” community on Lemmy.world. Lemmy.ml could (and possibly does) have it’s own Fediverse community. That would be separately run with separate content to the Lemmy.world Fediverse community.
Where it gets a little confusing, is that users in each of those different instances, can access and participate in the communities in each other’s instances. IE, if you set up your own Lemmy instance called TimeLighter.IsCool and created a community called “Timelighter appreciation society” I could potentially join that community using my Lemmy.world account (assuming you allowed it.) I wouldn’t need to create an account specifically on the TimeLighter.IsCool Lemmy to access it. If I did though I’d still (in theory) be able to use it to participate in the communities here at Lemmy.world.