Hexadecimal notation (or hex for short) is a numeric system where the base number is 16 rather than 10 in common math. Hex adds the symbols of A-F beyond the counting symbols of 0-9 to it's digit system. This means that 10 displayed in hexadecimal is actually equal to 16.
Hex digits are 4 bit values. Two hex digits placed next to each other represent an 8 bit value (0-255).
As they are practical for saving space, containing 255 diferent values inside 2 digits, they are commonly used in many trackers. They are used to write down the values of the volumn column, for example, and index the whole set of effects in one digit.