The RGB color space is reserved for sources that emit light and is based on the additive color model. This means that the more color you add, the closer to white you get. It is the same principle that governs a rainbow. If you were to add up all the colors of the rainbow in equal parts and to their maximum luminosity, you'd end up with white light. White light is just the sum of all the colors. In RGB color, it's each channel maxed-out.
When you open up an image in Photoshop, or any image editor, you can view the image's color information. This range is unaminously designated per channel as 0 to 255. Zero is no light and 255 is maximum light. If R=G=B at 255/channel you get white. The different colors you see are merely different number combinations.
The RGB color space has three main categories: