ELI5

Lemmy is a website where you can find interesting things to read and talk about with other people. It’s like a big library where you can find books on any topic you like and talk to other people who like the same things as you. It’s free and safe to use because there are no ads or bad computer code trying to trick you to stay longer on the site.

A bit more complex explanation:

Lemmy is a link-aggregator and the Fediverse’s replacement for Reddit. It is similar to sites like Reddit, Lobste.rs or Hacker News. You subscribe to communities that you’re interested in, post links and discussions and vote and comment on them.

It is a self-hosted social link aggregation and discussion platform that is completely free and open, and not controlled by any company. This means that there is no advertising, tracking, or secret algorithms. Content is organized into communities, so it is easy to subscribe to topics that you are interested in, and ignore others.

Lemmy is part of the Fediverse which allows users from other platforms to interact with posts created by Lemmy users. ActivityPub is the protocol used to allow Lemmy instances to operate as a federated social network and allows users to interact with compatible platforms including Mastodon, Calckey, and PeerTube.