If you mean lemmy as the name of the website then yes. All these instances(or servers) create the whole network which lemmy is trying to build. For example you signed up with lemmy.ml, an instance that was created and run by the lemmy devs. This instance has communities(like subreddits) where people can post things. You can sub to these communities and they'll appear in your feed. But other instances like beehaw also have their own communities where people can post, and you can subscribe to those communities too(using your acc here on lemmy.ml, without signing up on beehaw)

lemmy and beehaw have their own list of members and their own admins with their own rules of whats acceptable and not.

When you become more comfortable you'll realize lemmy can also interact with other fediverse software( or websites) like mastodon but for now this is more than enough.