Your starting image in Luminar is called ‘Layer 0.’ But you can also open up an entirely different photo, and add it as a layer on top or below your original photo. In this case, this image could be your desired background. You can then edit each image separately using Luminar’s presets and filters.
Adding a mask to a layer simply enables you to create different areas of transparency. This allows you to reveal parts of the image layer below. Think of it like tool that can create eye-holes in a mask, to reveal the person underneath. Or in this case, the layer below that you want to reveal for your background.