Hexadecimal is a way of representing numbers in base 16. Normally we use decimal numbers (base 10), but a computer uses binary numbers (base 2) which can be either 0 or 1. For convenience, computer programs and data are normally converted and displayed in base 16. Hex uses the letters A to F to represent the numbers 10 to 15 and if you ever see a number with an h after it, such as 45h, this means that it's in base 16.
