You made an account on lemmy.ml which means you automatically can check out the communities on this instance and through the federation. You can subscribe and treat this instance just like any normal forum/reddit type thing.
A quick note, lemmy.ml is the instance itself. When you see asklemmy@lemmy.ml or Linux@lemmy.ml the XXX@ is the name of the community inside that instance. Also lemmy@lemmy.ml is essentially the "homepage" community for the instance. Like a webpage almost. So if you see Android@lemmy.world that is the Android community on the lemmy.world instance.
However this is then where the "federation" comes in.
We're part of the "federation" of servers, meaning by default you can participate in "linked/federated" instances and subscribe/post/comment within them. Think of this like reddit1, 2, 3, and so on all exist. You're on 2 but can utilize all parts of 1, 3, etc from the same account. It's like your account is a general purpose account allowing you to contribute to multiple instances separate from your "home instance" which in your case is the @lemmy.ml part of your name.
Go to Communities and you'll see "Subscribed, Local, and All".
Subscribed is the ones you've subscribed to, ground breaking.
Local are the communities within the instance you're currently accessing through, so for you most likely it'll be lemmy.ml communities but for me it's lemmygrad.ml communities.
And "all" is the list of communities under all federated instances organized by (I think) user count/month. These are all open for your contribution.
However to all this I will say there are some caveats. Firstly some instances (beehaw for example) has defederated lemmy.ml. Therefore, if you're a beehaw user you can't see lemmy.ml content. However, do to the way the federation works us here on lemmy.ml can see beehaw content if you go to access it since "defederating" only works one way.
However, the content won't load by default or be stored on this server unless someone requests/accesses it first. At the time of requesting the information it'll download a local copy of that post to be stored on the "requesting" instance. (I believe this is the correct way of describing it)
However, contributions you and others make will stay only viewable to the lemmy.ml instance and will not be sent back to beehaw.
There is more technically information behind this but this is just a starter and over time you'll come to understand the interesting quirks of the tech.
Additionally there are admins of the Instances (lemmy.ml, lemmy.world, etc) and then mods of the communities. They're separate and you may be blocked from a community but not the instance. So keep in mind to double check rules whenever you are on a new instance/community.
Sorry if it seems like a lot but just trying to help out without making it confusing, I'm new here too haha. Best of luck and welcome to lemmy!