I was a bit confused about the place at first too. Here's a comment I copy-pasted from a previous post of mine:

My friend gave me a great explanation:

• Lemmy the platform is planet Earth

• “Instances” like lemmy.world, lemmy.ml, beehaw.org, etc. are like the different countries on Earth

• When someone signs up, the user picks one instance to be a part of, like how an Earthling becomes a citizen of a country

• If you register at lemmy.world, that means your home instance/ “home country” is lemmy.world, but you can “travel” to lemmy.ml, another instance / “country”, to check out and subscribe to their community

• When you subscribe to a different instance that’s not your home instance, you can still participate in their content, and other people will be able to see which instance / “country” you’re from

• Each instance can have its own version of the same “subreddit”, so you can have a c/Memes in your home instance that is different from a c/Memes in another instance. But you can subscribe to both separately

• c/[community name] is the naming convention used here I think like r/[subreddit name] on Reddit. If talking about a community in a different instance, it's c/[community name]@[instance name] so like c/memes@lemmy.ml

• Donations will help with the cost of running lemmy.world only and not lemmy.ml, beehaw.org, etc.

Someone please correct any of this if any of it is wrong, I’ll happily edit

Hope it helps even a little