Gesture Input works hand-in-hand with Gaze Input. If you think of Gaze Input like a mouse cursor, Gesture Input is how you "click" in HoloLens—which Microsoft calls "tapping." It's kind of like a touchpad, only in 3D.
You'll be able to use several gestures in addition to regular tapping, like tap and hold, zoom, and rotation. Think Minority Report comes to life.