These three letters stand for the RGB color space typical used in computer graphics. R means red, G means green and B means blue. This color space is typically used in computer graphics because a color monitor creates its colors out of these three components. A monitor is built-up by an ordered pattern of small red, green and blue colored segments. If these segments have a different brightness, different colors are resulting. For example, if blue is dark and red and green are shining, the result will be yellow. Because adding the base colors creates the various colors, the RGB color space is called "additive".
With the RGB color space, you can display 16.777.216 distinct colors. This value results from the fact, that one Byte is used for each color component (red, green, blue). One Byte is a basic memory unit of the computer and can contain values from 0 to 255. To calculate the maximum number of colors, you have to use the term:
256 x 256 x 256 = 16.7 mio. colors