There's already numerous apps to map our immediate environment in 3D with current smartphones, but this new Google project goes beyond, and because it's Google, the adoption potential of obviously significantly increased. MacRumors offers a nice summary along with several links in a story named Google Launches 'Project Tango' 3D-Mapping Smartphone.
From the official Project Tango site: "Our current prototype is a 5” phone containing customized hardware and software designed to track the full 3D motion of the device, while simultaneously creating a map of the environment. These sensors allow the phone to make over a quarter million 3D measurements every second, updating its position and orientation in real-time, combining that data into a single 3D model of the space around you. […] What if you could capture the dimensions of your home simply by walking around with your phone before you went furniture shopping? What if directions to a new location didn’t stop at the street address? What if you never again found yourself lost in a new building? What if the visually-impaired could navigate unassisted in unfamiliar indoor places? What if you could search for a product and see where the exact shelf is located in a super-store?"
Here's the 3-minutes video introducing Project Tango.