We commonly use a decimal or base 10 number system. We have 10 numbers (0 to 9), once you count past 9 you start over (the next number, 10 is composed of the a 1 to indicate we've rolled over once, and 0 to start using the numbers again).
Hexidecimal is exactly the same, with one difference, instead of using 10 numbers, it uses 16 numbers. While you could make up some new figures to represent the numbers past 9 it's a lot more convenient to just use regular letters to represent those numbers. So for the missing six numbers we can use the letters A to F.
| Decimal | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hexadecimal | 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | A | B | C | D | E | F |