Google launched the +1 button before it even launched Google+. It's always been a way to send social signals to Google, and help it determine what content people like. This helps the personalized search experience for one thing.
But while the +1 button has long given you the option to post a piece of content to the Google+ stream, just +1ing something without specifically sharing it publicly or with a particular person or circle has been pretty much guaranteed to go unnoticed by others, unless they happened to be digging around your profile looking at your +1 list or see it come up in their search results.
Now, Google is going to start highlighting +1'd content in the stream on its own. Google will "occasionally" highlight posts that were +1'd by people in your circles, and highlight your +1s to your friends.
This is pretty good news for bloggers and publishers because it gives content a greater chance of reaching more eyeballs. You can always see how many +1's you get on an article or blog post, but you never know how many of those are actually being shared to people. You probably still won't know, but at least you'll know that Google might put the post in more people's streams.
As Google's Shimrit Ben-Yair, who announced the new feature on Google+, notes, users can control who sees their +1s highlighted in the stream using the settings. If you don't want anybody to see them, you can adjust the settings accordingly.
"In all cases: friends will only see +1s and posts they already had permission to see, we’re just calling extra attention to them," says Ben-Yair. "And of course: if you’re seeing too much activity from a given circle, you can always turn down its volume, or exclude it from Home entirely."
The feature will be rolling out over the course of the next several days.