All activity is effectively sorted underneath topics aka “communities”. Generally the purpose of Lemmy is to post links or text/image posts, then users upvote and downvote for them to be higher or lower on the front page of the topic. As the posts decay, they downrank themselves automatically so newer content can replace it. It’s not really social media per se, it’s more like “hey look what I found related to this topic” and people can go “ooh” and “aah”.