You mean the mask.png? It is either black or white depending on what you want. I forget which one is which, but basically you use it to visually "remove" a charactewrs hair from specific areas of the sprite. So you can make like a had, but have your characters hair show under it, but not over it. That way the had doesnt look like its just resting on the hair but actually on the head pushing the hair down. So something like sunglasses could be all... black? A head might use a mix of black and white to make the above illusion. A full head helmet might use all white to remove any hair from view.
The mask is 43x43. Typical clothing sprite size. If you recall the sprite sheet for head items is also broken into 43x43 sections, so the mask basically should "overlay" with the sprite.
