Hex, short for Hexadecimal, is a base 16 numbering system. In your everyday life, you're used to a base 10 numbering system, beginning at 0 and ending with 9. You're probably aware of binary, which is a base 2 numbering system that only has two values: 0 and 1.

Hexadecimal has 16 individual values, going from 0 to 9, and then from A to F. Mathematical operations work exactly the same in hex as they do in decimal; they just use a different numbering system. For example, using decimal, you know that 9 + 1 equals 10. Using hex, 9 + 1 equals A (F + 1 equals 10).
Here's a quick table to help you understand:

Dec	0123456789
Hex	0123456789
Dec	10 11 12 13 14 15 16 17 18 19
Hex	A B C D E F 10 11 12 13

In the decimal system 11 is one ten and 1 unit. The number has exceeded the range 0-9 so we start all over again with one 10 and one unit.
Decimal 17 is one 10 and 7 units, or one 16 and one unit - the hexadecimal system, the number has exceed the range 0-F. So decimal 17 is hex 11.

The next piece of the puzzle is ASCII (American Standard Code for Information Exchange) character codes. Every textual letter, number, and symbol stored in a file or displayed on screen is an ASCII character; even the text you're reading right now is comprised of ASCII characters. Each ASCII character has an ASCII code, for example the letter "A" has a decimal ASCII code of 65, the letter "B" has a code of 66, and so on.