Lemmy, like kbin or mastodon, is a frontend to federated social media, ie ActivityPub. Lemmy is made to feel more or less like Reddit, while Mastodon is made to feel more or less like Twitter, and kbin is sort of in the middle and feels oriented to news aggregation, but they all interoperate over ActivityPub.

"Federated" means someone can pop up an instance, and even if the server isn't hosting any content, it can act your "view" into the fediverse.