Masking refers to hiding parts of a layer, usually a video or image.
Masks are specially useful for giving custom frames or shapes to dynamically loaded images, such as users avatars or posts media, where you don't have the possibility of editing those images. It is also very useful to give your own images a custom look, without having to edit the original images themselves.
You will find the Mask setting in the Appearance property of several Building Blocks:
Only available for Containers, this type of Mask will hide every part of all Container's descendants that overflow the boundaries of the Container itself.
This type of Mask uses a gradient as masking layer.
Different to the background gradients, the mask gradient control won't let you change colors, but only the opacity of each step of the gradient. This is because colors won't do any difference: the important thing is always the opacity.
This type of mask uses an image as masking layer. More specifically: it uses the image's alpha channel.
The mask image will work as long as it has transparent areas. Opaque pixels of the image will become the visible parts of your element, and transparent pixels will be hidden.
You can configure the same things regarding the mask image as you can with the background images: covering mode, tiling and position.