RGB is a pair of three colors, Red, Green and Blue. Each one has a value between 0 and 255, where 0 means off and 255 full on. Each number is then encoded into two digit hexadecimal representation from 00 (decimal 0) to FF (decimal 255) and simply concatenated with the prefix #.
Red for example is R=255, G=0, B=0, or in hexadecimal format FF, 00, 00. In RGB that is #FF0000