Boost is a repost or retweet on mastodon, and in the microblog sections of kbin.
On thread comments, a boost pushes the top level comments to the top of the stack. It means "this is the shit, everyone's gotta read this first."
On the threads view, a boost pushes a thread up the list but not all the way to the top, so it's practically an upvote.
In all cases, if you have followers, it pushes the threads and microblog posts you boosted into their feed.
One thing to know is, the up and downvote buttons don't do anything to the placement of a comment or post. It's a general like-dislike marker.
If you want to upvote something as in the reddit functionality, boost it.