are the coordinates of the source of the light ray which hits the camera sensor in world coordinates, relative to the origin of the world. By dividing the matrix product by , the z-coordinate of the camera relative to the world origin, the theoretical value for the pixel coordinates can be found.
This passage describes a method for determining the location of a pixel on a camera sensor, using the coordinates of the light ray source in world space. The calculation is performed by dividing the matrix product by the camera's z-coordinate.