The RGB color space is an additive color space which models the way light of a few specific frequencies are combined when travelling in the same direction. RGB is known as an additive color space because when light of two different frequencies travel together, from the point of view of an observer, these colors are added to create a new color experience. In particular, the colors which the RGB color space concerns itself with are the red, green and blue frequencies (hence the RGB acronym). These three frequencies are chosen because each one corresponds with one of the three types of color-sensitive cones in the human eye. With the appropriate combination of red, green and blue light, one can reproduce any color which can be experienced by a human. For more information on how humans percieve color, see the entry on color itself.
One common application of the RGB color space to display computer graphics on a monitor. For each pixel in the image, independent values for red, green and blue are stored. There is a fixed range of red, green and blue intensities displayable by the computer monitor, but by choosing the appropriate combination of red, green and blue light, one can therefore generate any color percievable by humans which happens to be between the black level and white point of the monitor being used to display the image. Typically the values stored vary between zero and 255.
As an example:
* (0, 0, 0) is black * (255, 255, 255) is white * (255, 0, 0) is red * (0, 255, 0) is green * (0, 0, 255) is blue * (255, 255, 0) is yellow * (0, 255, 255) is cyan * (255, 0, 255) is magenta
It is important to note that the intensity of the color output on computer display devices isn't always linear. That is -- even though a value of 127 is very close to halfway between zero and 255, the color output of a computer display device when displaying (127, 127, 127) is measured, you would likely find it to be significantly higher than halfway between the black level and white point of your monitor. This is because most display devices have a display gamma. (i.e. the behavior of most display devices is, unfortunately, not linear in the relationship between color value and output intensity).