When you render a pixel on screen (of a diffuse object), to determine how brightly it's lit, you need to compute the angle between the normal at the point on the surface and the light source. Computing this angle requires normalizing one or more vectors, which is done by dividing by the magnitude, or the square root of the sum of the squares of the components.

So the lights in this case are any lights in the scene, typically represented as a point of origin and a brightness.