Ad stacking is the process of placing multiple ads on the top of each other in a single ad slot. With ad stacking, only the ad on the top is viewable.
An other ad stacking practice is to place two or more ads immediately next to each other with no content between them. In this case, it is not really fraudulent but only poor ad placement because all ads appear in a cluster among others.
Sometimes ad stacking is used in the meaning of impression stuffing.