If you are referring to the overall look of a given body line along several panels, this is due to manufacturing sheet metal parts.
If you are talking ripples along a body line in a single panel, then that is indicative of a poor body repair at some time, or shrinkage of the repair material (bondo or primer).
When sheet metal is bent to form the skin of the door, fender or rear panel, it is usually a cold form press that forces the metal into shape. This method transfers the shape of the mold into the sheet metal via deformation of the metal and stresses the alignment of the metal molecules. This imparts added strength to the sheet metal thus formed as compared to a hot form process. The problem with this is that when the skin is then, once again, pressed around the frame of the door, or fender frame, or chassis, the original deformation doesn't yield in a uniform manner, and as a result the bend in the metal is not as sharply defined.
Almost all cars with the sheet metal bent around the inner frame work demonstrate this deformation, it actually shows up somewhat as a "bulge" 2-3 inches away from the frame member that continues all the way to the other member.
Look at a car from the side, and down, you'll note that the front and back part of the door, are relatively "straight" , but you can discern a slight bulge outwards by the sheet metal in the center of the door. The fenders and rear quarter panel will also demonstrate the same.
This difference in the surface protrussion can then be seen as mismatched panels or as reported earlier, a ripple.
All the posts above are each right in what they address.
One note, yes the best way to eliminate this and yield a front to back continuously flat / gently curved body is to use a high fill primer and to sand it with a long board. However, care must be taken not to overly fill around door openings, since these are usually lower than the rest of the body panel, it is too easy to leave them thick and then prone to edge chipping and damage.
For what it's worth