To add to that: On Lemmy and Reddit you care about specific topics such as Firefox@lemmy.ml or Science@beehaw.org. You don't really care who posts there as long as it's good stuff. People also vote the posts and comments so that trash sinks to the bottom while gold floats to the top.

On Mastodon (or Xitter) you care about specific people instead. You follow some people and hope they post good stuff. On Mastodon you can make list that collects together all the hashtags you care about. For instance, you could make a list about renewable energy, so you could throw in words like #solar and #wind. This way you could follow a specific topic. Either way, there's no voting, which means you'll just get a constant stream of anything and everything. People can boost posts, so if you trust someone enough to follow them, you'll also see whatever they boost.