SC Post Effects Documentation

Radiates sun shafts from the skybox at the position of the sun (or whichever object the SunshaftCaster is attached to). When no SunshaftCaster exists in the scene, the option will be displayed to add it to the Directional Light in your scene.

via Gfycat

This effect is a fast alternative to true volumetric lighting. Because it is a screen-space effect, sunshafts will only be visible when the SunshaftCaster transform is visible on screen.