What is Hex?

I've heard of this thing called hex, what is it, and where can I find it?

A Hex, is a color code.

When you go to the Colors section of the Admin panel, you use Hex Color codes to change colors within the forum.

I'll elaborate a bit...

Hexadecimal encoding (called Hex for short) is a code that consists of the characters A throughout F (including lowercase a, b, c, etc) and the numbers 0 throughout 9.
When used for color encoding, hexadecimals consist of either 3 or 6 combinations of the above, following the so called rgb system (red, green, blue), whereas they are preceded by a unique character, #, to initialize hex encoding.
Basically, a hex value #000 or #000000 (both are possible but the longer one is preferred) resembles the color black because of the rgb system which is basically the additive color mixture system (no values = no colors added to black), and a hex value of #FFF, #fff, #FFFFFF or #ffffff (the encoding does not care whether lowercase letters were used or not) resembles the color white due to the rgb system (full values = maximum amount of colors added to black = white).
The first two or first one (depending on whether you are using short forms) initialize a value for red, the third and fourth for green and the last two for blue. 0 is due to conclusion above the lowest value and F the highest, that's what you need to know. FF stands for 255, by the way.
I could go on but I think that's enough. If you want to know anything else, tell me