Base 16 math. Computers normally "think" in base two math, called binary. The only two digits are 1 and 0. So in binary the number 5 would be represented by. 101 (1 in the fours place, and one in the ones place.)

This binary thinking is generally translated into base 16 math. That is to say, you can have a single digit number as high as 15. Single digit numbers higher then 9 are usually represented by letters A-F.

So the hexadecimal number C5 translates to 197 in normal base 10 math; 12 in the 16's place and 5 in the ones place.