Sometimes, while editing a layer mask, we can't say for sure which areas are already masked and which are not.
To see the mask channel right in your image window, apply the "Show Overlay" 
effect.
In this mode, the mask is displayed as a 50% red overlay. This lets you scrutinize the mask and edit it more precisely.
To hide the mask and see the composite image as it is, use the "Hide Overlay" 
effect.
