Our usual number system is the decimal number system. As the name suggests, it uses base 10.
Using a certain base to express numbers means to write numbers as weighted sums of powers of the base. For example, in base 10 we write
So, the weights of the powers are what we call the digits of the number.
Note that our digits finish at 9 because it wouldn't make sense to have a digit to express 10: if we choose a weight of 10 (or more) for a certain power, we can simply scale it to the next one: for example, saying "ten hundreds" is the same as saying "one thousand":
Now, the hexadecimal number system simply uses 16 as base, instead of 10. This means that its digits are
1,2,3,4,5,6,7,8,9,A,B,C,D,E,F
where digits from A to F represent numbers from 10 to 15.
These are some examples of hexadecimal numbers turned into our usual system: