Oops, I'm still trying to figure out the formatting for links. Sorry!
The full URL should look something like yourinstancename/c/communityname@articleinstancename. So since you're on reddthat.com, for you to access ![email protected], you'll need to go to reddthat.com/c/[email protected]. If you were visiting a community that's on your own instance, you can drop the @instancename part at the end.
For what instances are, think of it like email. You can have Gmail, Outlook, Hotmail, AOL, or any other email client and can send messages to users on any other client since they share the same protocol/language. It's the same idea here. Every community is hosted on a particular instance, such as reddthat or lemmy.world or sh.itjust.works. Since all instances use the same protocol, a user from any instance can see and interact with content on any other instance (with an exception I'll get into in a sec).
Back to the email analogy. If someone using Hotmail gets an email from someone using Gmail, the Hotmail user doesn't actually access Gmail itself to read the message. Instead, Hotmail makes a copy of the message on its own servers for the Hotmail user to read. With the Fediverse, same idea - if you see content originally posted on another instance, you're technically seeing a copy of that content hosted on your instance. And if you interact with it, like making a post or commenting or even upvoting, you're doing that on your copy of the content, which is then synced back with the original copy. From there it's pushed out to all the other copies on all the other instances that are synced to the content. This is what Federation refers to - separate instances hosting different content that all communicate with each other to make a single community out of all its different parts.
The exception to all this direct communication is defederation, which is when two instances don't talk to each other directly (usually one cuts off the other). This means that users on those instances can't see or interact with content on the other instance. Defederation is a pretty extreme measure and its use varies instance-to-instance based on the admins. Some instances are pretty quick to defederate (such as Beehaw) but most see defederation as a last resort.
I hope that made sense!