It’s basically the ability to search amongst your friends.
Graph Search and web search are very different. Web search is designed to take a set of keywords (for example: “hip hop”) and provide the best possible results that match those keywords. With Graph Search you combine phrases (for example: “my friends in New York who like Jay-Z”) to get that set of people, places, photos or other content that’s been shared on Facebook. We believe they have very different uses.
Another big difference from web search is that every piece of content on Facebook has its own audience, and most content isn’t public. We’ve built Graph Search from the start with privacy in mind, and it respects the privacy and audience of each piece of content on Facebook. It makes finding new things much easier, but you can only see what you could already view elsewhere on Facebook.
I’m dubious of the privacy claims and the feature isn’t available on mobile apps yet. But, if you aren’t building for mobile first at this point as a developer then you are woefully out of touch with reality. Facebook should consider changing it’s motto to: “Making it even easier to stalk old girlfriends.”