Lemmy is similar to sites like Reddit, Lobste.rs, or Hacker News: you subscribe to forums you\’re interested in, post links and discussions, then vote, and comment on them. Behind the scenes, it is very different; anyone can easily run a server, and all these servers are federated (think email), and connected to the same universe, called the Fediverse. For a link aggregator, this means a user registered on one server can subscribe to forums on any other server, and can have discussions with users registered elsewhere. The overall goal is to create an easily self-hostable, decentralized alternative to Reddit and other link aggregators, outside of their corporate control and meddling.
A trade-free reddit, since there are no ads, trackers or other forms of trade. The beautiful thing about that piece of software is that you can host it on your own server and it is even connected to the Fediverse. You can find instances to join Lemmy here: https://join.lemmy.ml/join/