It’s an alternative to Reddit in the Fediverse.

The Fediverse is a network comprised of such software (Lemmy, Mastodon, PieFed, PixelFed, etc.)

Federation is the ability for different platforms to communicate using an agreed upon language (in the Fediverse, this language is called ActivityPub).

So if Instagram and Twitter were federated, you’d be able to post your photos on Instagram and have them also be on Twitter, all without having a seperate account at Twitter. All the likes and comments on Twitter will be available on Instagram and vice versa.

And because of this, servers (or instances of platforms) are not really dependant on each other. So even if lemmy.world were to shut down, every other instance will still operate just fine. For example, I run my own Lemmy instance lemmy.asudox.dev. Even if every other Lemmy instance were shut down, my instance will still continue operating without any issues. Any new instance can then link with my instance and start federating. The power of decentralization.

A nice video explaining the Fediverse by the Peertube team: https://framatube.org/w/9dRFC6Ya11NCVeYKn8ZhiD

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The video and my explanation are over simplifications. Federation will not work between all instances the same. Mastodon and Lemmy don’t federate fully. For example, Mastodon has no communities. Mastodon users just post (or “toot”, like “tweet” in twitter) on their profile. So you can’t subscribe (or follow) users in Mastodon from Lemmy nor see their posts, unless they post it in a community (which appear as just another user in Mastodon).

If you want to know more about the technical details, check the specification for ActivityPub at W3C.