3D Touch is the name Apple has given to the force touch-like technology in its iPhone line. On the Mac, Apple calls the technology Force Touch. It works through a sensor array beneath the screen (or trackpad, on the Mac) that measures microscopic changes in distance as the pressure from your finger increases. Rather than just being able to tap on an icon (or the screen) for a single action, 3D Touch allows for a whole new set of actions based on how much force is applied.
Here you'll find everything you need to know about 3D Touch, so don't go too far!