RGB refers to the different primary colors that are available. The primary colors are Red, Green, and Blue. These are the colors that are commonly used in various television screens, monitors, scanners, and even digital cameras. When you say CMYK, this still refers to primary colors, but this time, this would refer to the primary colors of pigment.
These colors of pigment are the following: Cyan, Magenta, Yellow, and Black. When you try to create RGB light will create white light while the combination of all the colors of the pigment will form black. It might be confusing to understand this in the beginning, but the more that you know, the better that it will be for you.