RGB color is based on the light spectrum, and it breaks color down into an RGB representation. All color can be defined by a certain amount of R (red), G (green), and B (blue). Light emitting devices such as TVs and computer monitors function in this manner. If you were to turn off your monitor, you would see black because no R, G, or B colors are present. This would be written as "RGB% 0,0,0". If you were looking at a white screen, you would be seeing "RGB% 100,100,100" because each red, green, and blue light source is shining at full potential. Other colors are created by combining varying amounts of R, G, and B.
True white light is composed of a full spectrum of all light colors, but RGB is close enough for most standards.
RGB is an additive color space because you get white by adding all colors together.