Part of making textures work in a photo is being able to control how present the texture’s detail is in particular areas of the image. Dirty Pictures, by default, will set up your image with a separate layer, which you can use to withhold the “gritty” parts of the texture in areas where you don’t want it to apply.
The Create Masks checkbox controls whether a separate layer and mask will be created to help control the texture. If you just want the texture by itself, uncheck this box, otherwise, leave it checked.
Dirty Pictures can configure the layer mask in one of two ways – either you paint white onto the mask to remove the texture, or you paint black to remove the texture. The default is to paint white, as the rest of the Totally Rad products use that convention, but if you think that’s backwards, and an abomination, then you can choose to have black be the color you paint to remove the gritty parts of the texture.