The analogy people are using is email. If I have a gmail account and you have a yahoo account, we can still communicate between each other, but I cant use my gmail account to log in on yahoo's website. Lemmy works in a similar way. You make one account on your instance of choice (I see you chose beehaw.org) and then you can subscribe and otherwise interact with things and people on other instances (servers).
Here's how you can do that. Go to your server's search bar, and type in !dnd for example. Give it a few seconds and it should pop up as an option to click on it. Once that happens you can subscribe to it from your instance and interact with it normally.
I hope this helps, and if you have any other questions dont hesitate to ask me or anyone else. Most people are quite helpful :)