To quote the Lemmy project1)
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.
As Lemmy is part of the Fediverse you can subscribe to Lemmy forums, start new threads in the forums and comment to threads of others.
Some Lemmy instances perform allowlisting of servers they federate with. If the forum you want to join is located on an instance that does so, you have to contact their admin team to include your Friendica instance into the allowlist.
If you want to start a new thread on a Lemmy forum you address it like forum in Friendica by adding the handle with the !bang notation. Additionally you have to set a title for the posting.
Once you have joined a forum, it will be shown in the _Forum Widget_ in the Network stream of your Friendica account. Once new threads are created at Lemmy, they will be distributed to your Friendica account just as other postings from the Fediverse. You can either interact with these postings from your network timeline or select the forum from the widget and interact with the postings from that stream.