Masking is when you want to exclude part of the image from processing or analysis. Usually this means that you set the part of the image not in the mask to zero, but can have other meanings depending on what you want to do.
The code you are referring to makes the mask the same class as the image because if the image is uint8, uint16 or some other integer, you can't multiply it by 1 or 0 unless the 1s and 0s are the same class. Then the rest of the function just multiplies the mask by each color channel in the image. If the mask = 1 the original pixel remains, if the mask = 0 the pixel is set to black.
