Just ignore the number after the X. You don't need it unless you are writing programs.
But to answer your question, Hex uses 16 numbers instead of 10.
1,2,3,4,5,6,7,8,9,A,B,C,D,E,F,10,11,12,13,14,15,16,17,18,19,1A,1B,1C,1D,1E,1F,20 etc.
11 in hex would be 17 in decimal.
We use the decimal system because we have 10 fingers and that's how we developed math.
Computers use binary (base 2). 1, 10, 11, 100, 101 etc.
As you can see, the numbers get very big very fast and so would be too bulky for us. Octal (base 8), and hexadecimal (base 16) are very easy to convert to binary, even in your head. Decimal is not. So computer code is written in Hex.