Lemmy is like if email and Reddit had a love child. With email you can have different providers (Gmail, Yahoo, Hotmail, etc) that all talk to each other (Gmail accounts can send emails to Yahoo accounts, etc). Lemmy is like that. There are different instances (lemmy.world, sh.itjust.works, lemmy.ca, etc.), and they are hosted on different servers, by different people, but they can all talk to one another Reddit style (they upvote each others posts, respond to each others comments, etc).

The benefit of this is that it’s decentralized. If you are having an issue with your Lemmy instance, you can just go to a new one and take all your account settings with you (compare that to Reddit, which is very centralized. If you have a problem with how its run, then you either have to suck it up or leave the platform entirely)