hex is the abbreviation for the word hexadecimal
and hexadecimal is a counting system were every individual digit can hold 16 possible values.
Instead of the decimal type, the one you are used to, that only holds 10 values (0 to 9)
in hex it is (0 to F)
0 1 2 3 4 5 6 7 8 9 A B C D E F
then to show a greater value it would go on to... 10 11 12 13 14 15 16 17 18 19 1A 1B 1C 1D 1E 1F
and now.. for example
7B in hex = 123 in dec
with hex you only need 2 digits, with dec you need 3 to show the SAME value