It's a way to manually trigger the Auto Awesome Snow effect for any photo.
You need the latest version of the Google+ app (4.2.4), which was released today. Blame staged rollouts if you don't have the new version. You can also manually download the APK file.
This is not the only new feature: "when you +1 posts in the stream, you'll see something lovely" (hearts). There's also a unified search box, a new section for less important notifications and separate "What's Hot" streams for different categories.