Earlier this year, Google showed off its new conversational search capabilities for the desktop, including the ability to perform a search by simply saying, "Ok Google," and then your search phrase.
In May, Google released conversational search for Chrome, but it didn't have the "Okay Google" feature. The company has now made that available, but you have to download a Chrome extension to take advantage. Google announced it in a Google+ post:
The feature is kind of cool in a "wow, look what I can do" kind of way, but I'm not sure it's entirely useful, considering that you have to go to the Google homepage (it also works from the new tab page) to use it. If you're on another site, you have to navigate to Google, so you might as well just type in your search in the omnibox.
But hey, look what Google can do. I guess it's a reason to keep Google open all the time. Well played, Google.