A Image or layer mask is the use of masks so that only certain parts of the image are shown. For example, a shape would be drawn in the base layer this is known as the mask (the layer right at the bottom) after that all layers and images placed above that layers would only show the portions of the image that fall within the area of the Mask and the parts of the images that are outside of the mask would be hidden this is known as a clipping set.
Image Masks can be used to create very powerful visual effects, from techniques such as image blending to stylizing texts. Image Masks can be used to make some bold statements. Image blending refers to the use of two source images which are blended into one. Stylized texts use fonts and textboxes as the mask, so the text is in the base layer and the images are placed over it. All images placed on it afterward take the shape of the text at the base of the image. This can be further improved by adding a different background than that of your clipping set and from the use of alpha channels.