"Whenever you have a new notification (for example, when someone adds you to a circle or shares a post with you), the extension button will turn red and show you how many updates are waiting for you. Once you click the button, a window will drop down showing you all of your most recent notifications. You can act on notifications right from the extension, whether you're replying to a comment or adding someone back to your circles. That way you can stay up-to-speed with your circles on Google+ no matter where you are on the web," informs Google.
There's also a +1 button for Chrome that lets you +1 a page and share it with your circles.
{ Thanks, Venkat. }