It’s just another Mastodon-compatible server where you can log in.

Mastodon, like Lemmy, is a federated network, meaning the more servers there are, the better things are for everyone. Huge servers with millions of users are harder to maintain, and moderation does not scale. There’s more noise, abuse is harder to spot, and harassment is harder to punish. Having many small servers is a better approach.

On the other hand, new servers might be unstable: the admin needs to both maintain the server and moderate the community, and there have been cases of admins doing crappy jobs at either. Servers are born and die regularly on Mastodon; you might want to see how things go before opening an account there.