The letters RGB stand for the colours Red, Green and Blue. These are the 'primary' light colours which come together to make other light colours when seen by the human eye. For example, 'White' light is made up of Red, Green and Blue light.
In these types of light bulbs, Red Green and Blue LEDs are used together to produce a vast array of possible colours, using the same principle detailed above. RGB LED lighting systems are often used for lighting applications such as Disco, Party and other occasion lighting. RGB LED Lighting systems can provide you with some unique, coloured lighting effects.