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.

Lemmy is a social platform for people to share content and links and talk and discuss. Lemmy uses communities to group and host people and content to for themes - like animals, or pictures of cats, or videos of kids falling.

Lemmy is a content aggregation server. It’s a piece of software that allows people to set up and run their own Reddit-like websites. By running Lemmy, you can host a website with open forum-like communities that are focused on a particular topic, place, region, or whatever you can imagine. Just like on Reddit.

One of the features of Lemmy is that it can speak a type of Internet language, called ActivityPub, that allows website running Lemmy to communicate with each other. This let’s them to share and cross-post content, and Lemmy gives website owners, and their guests, the ability to form alliances with other websites running Lemmy, where they agree (usually implicitly, but it can be explicit, too) to share content between them, letting people on both sites see what people on the other site are posting, and even to interact with them.

Lemmy isn’t the only piece of server software that speaks this language. It’s also spoken by kbin, which is a different content aggregator server, meaning Lemmy websites can also share content with kbin websites. And it’s spoken by websites running. Mastodon, Calckey, Misskey, Pleroma, Akkoma, Friendica, Hubzilla, PixelFed, PeerTube, BookWyrm, FunkWhale, and others, meaning peiple using websites running those pieces of software can also, at least in theory, see and interact with content on Lemmy-based websites, and vice versa.