White light can be thought of as a combination of wavelengths, containing all of the colors in the visible spectrum. Red light has long waves compared to blue light in the other end of the spectrum. When light interacts with a material, the light is partially absorbed by that material. If some wavelengths of light are absorbed and others reflected, the material will look colored by the reflected wavelengths. Absorb the short/medium wavelengths, and end up with only red light reflected, a red colored material. This is what we call subtractive color. Color you get from subtracting wavelengths from the incoming light, as opposed to Additive color, colors you get from adding light of different wavelengths to each other.
Examples of working with subtractive color is painting witch acrylic, oil or watercolor. You have an external lightsource, like daylight or incandescent and if you subtract all wavelengths from that you end up with black color. You can only take away color this way, revealing the remaining color in the light. An example of working with additive color is doing a picture in RGB on the computer, where you define how much red green and blue light is going to be seen at each pixel on the screen. If you add the maximum amount of red green and blue, you end up with white.