The Fediverse is called that because it is “federated”, meaning different instances (and different platforms too!) can communicate with each other. A Mastodon user can see Lemmy posts and interact with them, for instance, and in your case, you can see Lemmy posts as an Mbin user. Everything is interconnected rather than being confined in their own little box!

“Instances” are what we refer to the hosted servers. For example, sopuli.xyz, lemmy.ca, feddit.org, and programming.dev are all Lemmy instances, hosted by different people and groups around the world. These can all talk to one another, so even a tiny instance where the only user is you and a few friends can see the posts of everybody else, and vice versa! Just something to keep in mind, as Lemmy, Mbin, etc. are not one instance, they are collections of many!