Chromatic aberration is probably most easily understood as a prism effect. When light shines through a prism, you see that different colors of light bend differently and create a rainbow effect. When light shines through any lens, including glasses and the natural lens of your eye, the same occurs to a lesser degree. The effect of this is that the Focal length of the lens is not exactly the same for all colors of light.
The chromatic aberration expected during a normal eye exam is about a half diopter difference between red and green. This can be a handy test to dial in a final prescription by looking at text on red and green backgrounds and seeing which is clearer. If the green side is clearer, a quarter diopter of sphere is added, if the red side is clearer, a quarter diopter is removed. If the optometrist is trying to introduce a small amount of Myopic defocus he/she wants the red side clear.