RGB stands for red, green and blue, the three colour ‘channels’ that go to make up all the colours in a digital image. It comes in two varieties – sRGB is a ‘universal’ RGB that can be used and displayed by any device, whereas Adobe RGB is a more specialised alternative for pros.
The photosites on the sensor use red, green and blue filters to capture colour information and image-editors let you work with the red, green and blue channels. It’s an example of a ‘colour model’. Others include the CMYK colour model used by printers and the Lab mode used in video.