Hexadecimal is a numerical system with a base of sixteen unique digits.
Web colors can be defined by a hexadecimal system that goes from the number zero to the letter F.
0 1 2 3 4 5 6 7 8 9 A B C D E F
Count them! There's sixteen digits! Imagine that!
Okay now think of those digits as a sliding scale where zero is all the way "off" (or black) and the letter F is all the way "on" (or white).