is the spotlight's direction (GL_ SPOT_DIRECTION), assuming the light is a spotlight and the vertex lies inside the cone of illumination produced by the spotlight.
The dot product of the two vectors and varies as the cosine of the angle between them; hence, objects directly in line get maximum illumination, and objects off the axis have their illumination drop as the cosine of the angle.
To determine whether a particular vertex lies within the cone of illumination, OpenGL evaluates where and are as defined above. If this value is less than the cosine of the spotlight's cutoff angle (GL_ SPOT_CUTOFF), then the vertex lies outside the cone; otherwise, it's inside the cone.