The projector emits a light pattern. The most common patterns are planes and single beams. We shall use a projected plane for illustration. The intersection of the plane with the scene surface is a planar curve called the strip, which is observed by the camera. By using triangulation we get the depth map of the surface points under the strip.
Distance is obtained as a function of the time taken by the wave to hit a surface and come back.
Other phase difference sensors measure the phase shift of the observed return beam. Distance is a function of the phase difference. Once the best focused image is determined, a model linking focus values and distance yields the distance.