The person above is correct that is has been a thing since the 1950s, but for most of that time it was a relatively rare term used to refer to the UK's communist party (which hasn't even existed since 1991) and occasionally for hardline old-fashioned leftists in general, again mostly in the UK. It's only in the past couple of years that it got picked up by the internet and went everywhere, which is probably why you haven't heard of it. I'm from the UK myself and I haven't heard of it until recently.
If boosting is meant to be a solution to the back catalogue problem, then it's a horrible way to do it. You'd have to go through and boost every single post from before the hosting instance was followed, and then it'd only show up the user page of the guy who went to all of that effort? (or, realistically, bot).
If what I'm saying is accurate (and I'm still not sure because this is admittedly a bit too complicated for me) then it doesn't sound very useful since individual profiles aren't nearly as important in a forum context when compared to something like twitter, and especially when you can just upvote something and have that show on your profile. Unless I'm mistaken and anything you've upvoted doesn't propagate to another automatically instance while boosts do... but I don't think that's a big enough distinction to have two different buttons? You could just have an upvote also do that.
The best thing about Kbin is that we aren't reliant on the Lemmy devs
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PSA: while upvoting exists, to get the "move closer to the top" effect that reddit's upvote had, you need to click boost
a small difference, but important to how people use the site