It's a new extension for the Chrome browser. The way it works is that you'll be on a page and there will be a little box down at the bottom of the screen pointing you to links to other content that Google deems relevant. You're on a news story, it will show you related stories you can find. You're on a restaurant page, it will show you reviews or similar restaurants.
Because people love being completely distracted at all times!
Ars Technica says this is still a little buggy and the results aren't always so relevant:
What the extension lacks the most is the ability to train it. Links offered from the Related bar are +1-able, but if you click the "View More Articles" link from the story above, you get a get a long list of stories from various outlets that can't be +1'd. This strikes us as a prime opportunity to teach Google Related which sources you trust or would like to see in your related news tab when you visit a news story. Still, true to Google form, Google is collecting statistics on the project, so we may be training it more than we know.