Opacity maps create the illusion of opacity and transparency.
You can specify areas of opacity and transparency by using the opacity map channel. For example, when using a Realistic material type, if your image is a white circle in the middle of a black rectangle and you apply it as an opacity map, the surface appears to have a hole in it where the circle maps onto the object. When using a Realistic or Advanced material type, a black circle in the middle of a white rectangle would appear to have a hole in it where the circle maps onto the object. If the image is in color, the gray-scale value of each color is used.