Lemmy is a piece of software. Lemmy software is a link aggregator - same as reddit.

So you're signed up to an instance of Lemmy which is installed on the server at lemmy.ml - this means that the server you signed up to (lemmy.ml) is running a copy of the Lemmy software. Other servers also run the Lemmy software making them also instances of Lemmy. As well as you being able to talk to users in Communities on the lemmy.ml server, you can talk to users in Communities on other Lemmy instances. For example, I'm registered on the server at lemmy.world

KBin is also link aggregator software, just like Lemmy and Reddit. Same things apply there, same software on multiple servers, all able to talk with each other.

Mastodon software is a microblogging service - same as Twitter (and Threads). Just like instances of Lemmy, instances of Mastodon can talk to each other. So a user on mastodon.world can talk to (for example) a user on kolektiva.social which is also running the Mastodon software.

Plume is blogging software - like WordPress, but just like Lemmy and Mastodon, it can be installed on multiple servers, all of which can talk to each other.

There's also Pixelfed (Instagram), PeerTube (YouTube), Friendica (Facebook) and a large variety of others.

Now, as well as all these different types of software (Lemmy, Mastodon, KBin, PixelFed etc) being able to talk to other instances of the same software on other servers, because they are all underpinned by a single method of passing information called ActivityPub, each type of software can also talk to each other - so you as a Lemmy user can also see posts (like the one you and I are responding to) from a user on a server running an instance of Plume. Some people here are commenting from a Mastodon instance. All these things are loosely joined together making a joined (federated) universe - the fediverse.