Facebook has been working on getting people to pay to promote their posts, i.e. have them visible to more people than those who would normally see the update. The feature may never be a huge revenue stream for Facebook, but the site needs to be experimenting.
Now, it's pushing that experiment further by making it possible for users to pay to promote other people's posts, even without their approval. This only works for friends' posts, but it's still a bold move.
The feature is rolling out slowly globally. It means that, apart from promoting your own posts, you can promote your friends'. As for why you would want to do that, that's probably what Facebook is trying to find out.
You could use it to help a friend who's running a charity, doing something for a cause or simply looking for a place to stay.
Normally, anything you post is only seen by a few of your friends, 16 percent or so. That's because they may not be online around the time you post it or you don't interact with them all that often.
Facebook looks at a lot of data points to determine what you see in the news feed and what you don't, since it would be impossible and counterproductive to simply push everything in front of you.
However, if you have a very important announcement and want to make sure as many friends as possible see it, you can pay to have it promoted. It's only about $7, €5 depending on several factors.
This won't guarantee that everyone will see it, but it will put it in the news feeds of more friends than by default.
The same goes for friends' posts you promote. Both your posts and your friends' posts will respect the privacy settings, even if you pay the post won't be seen by someone that couldn't have seen it in the first place.