Hexadecimal
Humans mostly use the decimal (base 10) system where each digit can have one of ten values between zero and ten.  This is probably because humans have ten fingers on their hands. Computers generally represent numbers in binary (base 2).  In binary, each "binary digit" is called a bit and can only have one of two values: one or zero.   Since a single bit's two possible values represents one fifth the information potentially conveyed by of decimal digit's ten possible values, binary representations of integer values can require many more (binary) bits than decimal digits.