The red-green-blue color model is one method by which computers can represent color. It is an additive color model, where the values represent "how much" red, green, and blue light should be mixed to create the desired color--such that (0, 0, 0) is no light, and thus black, and (1, 1, 1) is all light, and thus white.
Additive color models, at a first approximation, are effective at communicating color data to an emissive display like an LED or a plasma screen. However, in its raw, three-tuple form, RGB is a device-dependent color model and suffers from significant representation issues; image and video solutions generally incorporate some kind of color management system to map RGB values to something more accurate.
The RGB color model is directly convertible to the HSV color model, and both are useful for different purposes.