"RGB stands for the three primary colors of light - Red, Green, and Blue. RGB can be described as the computer's native color space for capturing images and displaying them. As human eyes are sensitive to these primary colors - red, green, and blue - all colors are perceived as a combination of these three colors. The RGB color model, based on a Cartesian coordinate system, is considered as an addictive model in which red, green, and blue, are combined in several methods to reproduce all other colors. In the RGB color model, each color appears in its primary spectral component of red, green, and blue"
http://www.tech-faq.com/rgb.shtml
Basically, they are numbers that determine the color.