It is the waviness across the flat area's of sheet metal panel's. It it usually the most common in lighter gauge metal's. More apparent under shallow cross lighting so its presence is more visible during certain season's or time's of day. Oil canning is simply an aesthetic issue, not a structural problem or defect. Its quite unrealistic to expect any architectural roof or anything similar to be totally free of some degree of oil canning.
If your wondering how you can prevent Oil Canning,
Well here are a few way's to prevent it,
Material thickness and method of attachment, the thicker the gauge of metal the more likely you will not have a problem with oil canning.
Typical cause's are uneven stresses at the fastening point's.
The panel design that you choose as well has a lot to do with if it is more susceptible to oil canning.
The way you handle the material as well can also be a factor and most importantly the installation process.
Usually oil canning mostly occurs when using the light gauge metal panel with smooth panel profile's and flat surface's.
Panel's that have ridged shape's and narrower flat's are less likely to exhibit oil canning.