This occurs when a low definition picture, e.g. from a newspaper or a book, has been scanned with too high a resolution. The effect is interference patterns which look like dark bands across the picture.
This can be corrected by selecting "descreen" as an option when scanning. Otherwise, it is very hard to correct and the best option is to request a rescan at a lower resolution. If the effect is not too bad, using an image editing software to blur the photo may help.