Google+ is Google's social network. It's fully integrated into Blogger (there's a "Google+" tab on the admin panel). There are some cool benefits to linking your blog with your Google+ account. You can easily share your blog posts with your Google+ friends, and they can share them more easily as well.
It's also easier to see how people are reacting to your posts. You can see who gave your post a "+1" and see who shares your post (and who subsequently reshares it).
Google+ is, above all else, a way to stay in contact with friends and associates, you can have active discussions about your blog posts or even host Google+ hangouts and chat with your readers live.