Hexadecimal is a numbering system that uses sixteen symbols. The hexadecimal number system uses the same symbols as decimal (0-9), but because we humans count in decimal, we don't have symbols behond 9, so we switch to letters and keep going till we hit fifteen (A, B, C, D, E, F). When we hit fifteen (F) we flip over and add one to the next column.
Hexadecimal and the Nybble
You can represent one hexadecimal value with four binary bits. Four bits make a nybble and two nybbles make a byte. Therefore it takes two hexadecimal numbers to represent the value of one byte. Once you understand that eight bits make a byte, this makes sense.
Media Access Control (MAC) addresses are six bytes long and are written in hexadecimal. Here's an example of a MAC address written in hexadecimal and in binary. They both equal the same value, they are just represented using different symbols.