What Is It?
The new (coming soon) Google plus 1 button let's you recommend web pages by clicking their Plus 1 button. You can do this by clicking the Plus 1 button on the actual page or next to the page in a search result or next to a Google advert that leads to a page. Simple.
How Does It Add Value?
It adds value by showing other people in your (Google) social network that you are recommending a page/company/site. So if for example someone in your social network searches for a new kettle and they see you Plus 1'd a certain page in a search result that might help them choose their next kettle. Simple.
Anything For SEO's To Worry About?
Maybe, but only if you are spamming rubbish sites up the rankings :)