Its called "banding"
You usually see this in the shadow area of a photo (dark areas), and sometimes go hand in hand with a high ISO (just looking at your image it doesn't look like an incredibly high ISO was used). Without having the original image to look at (straight from the camera) I cant tell you for sure what the problem exactly is. Some cameras are just not as good at capturing shadow detail. Do you know the camera make and model name? Know what ISO you shot it at?