RGB (from Red, Green, Blue) is a color model - or color space - that consists of three primary colors (Red, Green and Blue, of course) which are added together at different proportions to create any other color. Thus RGB is an additive color model.
The number of bits used to provide R,G,B information (ie the color depth) determines the maximum number of color variations that can be represented by a specific RGB model. Typical values for modern video is 8 bits (one byte) for each R,G,B primary. Typical specific RGB formats for video are RGB32 and RGB24.