Metalness Mask is a Shader Mask available in Source 2 and Source 1 ($masks1 only).
The Metalness Mask reduces and darkens the color and rimlight in order to simulate a real-world metallic look. It works in combination with the Base Tint mask, which returns the color through specular highlights. This mask is used in combination with other masks that help the surface appear more metallic. Typically this means bright, broad highlights that are tinted by the base color.
The mask works as following, greyscale is used, where a full white means "no metal" and full black means "metal".