Red, Green, and Blue (RGB) are the primary colors used to display a game on your computer screen. All the colors you see are combinations of those three.
In games, we use numerical RGB values to describe the colors in each Texture. These numbers can be a bit more difficult for artists to understand, but they give more control especially when you want to tweak something like texture blending.
In texture programs like Photoshop, the RGB values for texture colors are in 8bit scale which is usually 0 to 255. But game engines prefer a simpler scale, representing all colors with the values 0 to 1. For instance, red is (1,0,0), white is (1,1,1), black is (0,0,0), brown is (.4,.21,0) etc. The decimal places can go out as far as needed to, but usually just two decimal places (.00) is precise enough. The less decimals, the smaller the file sizes will be, which conserves precious Memory.
See also AdditiveColorModel.