It’s a sequence of three hexadecimal numbers, two digits each. The first two digits denote the amount of red, the middle two green, the last two blue (#rrggbb).

0 to 9 represent the values 0 to 9, A is 11, B is 12, and so forth to F which is 15. For each of the three pairs of digits, you have to multiply the left one by 16 before adding the right one.

For example, 4A = 4 * 16 + 10 = 64 + 10 = 74. The highest possible value is FF = 15 * 16 + 15 = 240 + 15 = 255 and the lowest is 00 = 0 * 16 + 0 = 0.

#ff0000 is the maximum amount of red and zero green or blue, so a bright red. #ff00ff is the maximum amount of red and blue, but no green, a bright magenta. #8000ff is roughly twice as much blue as red, a shade of purple. #ff8080 is a pale red, #800000 a dark red.

If you need more explanation, look up hexadecimal number notation and hexcode colours.