Way of storing images.
RGB stands for Red, Green and Blue. The acronym can mean many thing but in context of high and standard definition television it almost exclusively addresses image storage and processing in digital format.
Storing images using the color primaries is as old as color photography and can be emulated digitally. In digital realm RGB takes advantage of a physical phenomenon called additive mixing. When color primaries such as red, green and blue are added in equal amounts white light is created. The white color coming off your computer screen is really a mixture of red, green and blue which can be easily verified with a magnifying glass. Varying the percentages of the three primaries creates other hues and values.
RGB is just one of color models used in TV and photographic imaging.