Mask

An image that defines areas in a frame to be used as a transparency key or matte. Each pixel in the mask image indicates the degree of transparency to be used for the corresponding pixel position in each frame. 

You can create two types of masks:

• Layer masks are resolution-dependent bitmap images that can be edited with the painting or selection tools.
• Vector masks are resolution independent and are created with the pen or shape tool.

Layer and vector masks are reversible; this means you can go back and rearrange them later without losing the pixels they've hidden. In the Layers panel, layer and vector masks are displayed as additional thumbnails to the right of the layer thumbnail.

For the layer mask, this thumbnail represents the grayscale channel created when you add the layer mask. The vector mask thumbnail represents the path that clips the contents of the layer.

To create a layer or vector mask over the Background layer, first convert it to a regular layer (Layer > New > Layer From Background).

You can edit the layer mask to add or subtract from the masked region. A layer mask is a grayscale image. Areas you paint black are hidden, areas you paint white are visible, and areas you paint in shades of gray appear at various levels of transparency.

A vector mask creates a sharp-edged shape on a layer and is useful when you want to add a design element with clean, defined edges. When you create a layer with a vector mask, you can apply one or more layer styles to it, edit it if needed, and instantly have a useful button, panel, or other Web design element.

The Properties panel provides additional controls for adjusting a mask. As in the selection area, you can change the mask's opacity, invert the mask, or thin the mask borders to increase or decrease the transparency of the masked content.

For more tips in creation process you can check our blog:

https://cinema8.com/blog/a-guide-for-adobe-rush