Mastodon uses the boost button as well. I think it’s similar to an upvote, it just kind of helps the post get more attention. Since kbin is a kind of hybrid of mastodon and Lemmy, it uses both the upvote/downvote buttons and boost button. I’m not sure if boosting Lemmy posts does anything though. I haven’t used kbin or mastodon very much so some of my info may be incorrect.