Lemmy is similar to sites like Reddit, Lobste.rs, Raddle, 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.
Each Lemmy server can set its own moderation policy; appointing site-wide admins, and community moderators to keep out the trolls, and foster a healthy, non-toxic environment where all can feel comfortable contributing.