A clipping mask is a layer (shape) to which a mask (image) is applied. For example, say you want to have a circle picture, but your picture is square by default. Just select a round shape from the Shapes tab in our editor. Situate your shape in the desired place in the flipbook, then check the Use as mask option from the top menu. Now your shape can be used as a mask, all that's left to do is for you to select the perfect image from the Images tab, drag it and drop it on your mask.
