This input window allows you to set the direction of a directional light. When setting the light direction, you can think of the coordinates as those of a point with the light going from that point towards the origin. For example, if we want the light rays to point more to the left and, in turn, the shadow move to the left, we increase the value of the x coordinate.
As for any of our tutorials having a parameter input window, we can use either the mouse or the up and down arrow keys to change any of the selectable values that are initially drawn in green.