"Mask" applies any orange preview layers and active selection/path as a layer/vector mask on the active layer. This is an ideal way to mask existing layers that were not created by Lumenzia, such as the output from Nik Color Efex. It can also be used to replace an existing mask.
The next video is not specific to the "Mask" button. It shows the masking options that will appear for Mask or any of the other solid orange apply buttons or Group when you have an active selection or path (because Lumenzia needs to know what you want to do with that input).