RGB (Red, Green, Blue) is a common way of representing colour in a digital image. The colour of each pixel is represented by three numbers, being the amount of red, green and blue light making up that colour. These are normally 8-bit binary numbers, and so range between 0 and 255 when written in decimal (or x00 and xFF in Hexadecimal)- as seen in colour selectors in photo processing programs. This leads to the most frequently used colour depth of 24 bits.
There are alternatives to RGB, such as CMYK - which is often used to specify colours to computer printers.
RGB is also a simple method of connecting computer monitors, where colour is represented by three separate analogue signals - and the computer hardware will often use RGB for colour representation.