For each pixel,
Intensity of light = gray level in gray scale images.
This is essentially the analogy in image analysis of the luminance  in optics. It runs between 0 and 255.
The intensity of a dark object is the lowest level of gray within it. The intensity of a light object is the highest level of gray within it.
For example, rectangle #1 has intensity 0 (black). The intensity of rectangle #5 is 255 (white).
Compare to Optical density.