It's RGB Code (Red Green Blue).

The first 2 numbers are how much red is in it.
The second 2 are the green.
The third 2 are the blue.

Different combos of letters and numbers make the percentage of that color.

FF=100%
00=0%

Thats why FFFFFF = white = 100% Red, 100% Green, 100% Blue
And 000000 = black = 0% Red, 0% Green, 0% Blue.

ff0000=red
00ff00=green
0000ff=blue

I hope you can understand it by now....