Red, green, and blue (commonly called RGB) are the primary color elements that make up white light. Because you ADD the colors together to get white light, we call these RGB colors the additive primaries. Colors on screen are displayed by mixing varying amounts of red, green, and blue light.
Adding any two of the additive primaries creates one of the subtractive primaries -- the colors used in 4-color process printing. Keep in mind that the additive primaries typically refers to the RGB on-screen color mode. Mixing actual red, green, and blue inks or paints does not produce white.