Upvote and downvote is just that (they recently fixed the reputation calculation). A boost is like a retweet. You crosspost the boosted content to your own microblog space. Kbin does both reddit-like discussion threads as well as twitter/mastodon like microblogging. In fact, kbin is fully interoperable with mastodon. By boosting something you retweet (retoot?) it to your mastodon/microblog.