What is a hexdecimal system?

Answer 1

That is "hexadecimal". The decimal system we often used is based on powers of 10 (each place-value is worth 10 times as much as the one to the right); the hexadecimal system is based on powers of 16, and therefore needs 16 different digits. The "digits" commonly used are the digits 0-9, the "A" for 10, "B" for 11, ... "F" for 15.

This is commonly used in computers, as a sort of shorthand for writing binary (base-2) numbers.