The hexadecimal numeral system, hex, is a numeral system made up of 16 symbols (base 16).

Your standard numeral system is called decimal (base 10) and uses ten symbols: 0,1,2,3,4,5,6,7,8,9.

Hexadecimal uses the decimal numbers and six extra symbols, from the English alphabet: A, B, C, D, E and F.

We mostly use the decimal system. This is because humans have ten fingers (ten digits). Computers however, only have on and off, called a binary digit (or bit, for short). A binary number is just a string of zeros and ones: 11011011, for example.

For convenience, engineers working with computers tend to group bits together. In earlier days, such as the 1960’s, they would group 3 bits at a time (much like large decimal numbers are grouped in threes, like the number 123,456,789).

Three bits, each being on or off, can represent the eight numbers from 0 to 7: 000 = 0; 001 = 1; 010 = 2; 011 = 3; 100 = 4; 101 = 5; 110 = 6 and 111 = 7. This is called octal.

As computers got bigger, it was more convenient to group bits by four, instead of three. The additional bit can be either on or off, a 0 or a 1. So this doubles the numbers that the symbol would represent. This is 16 numbers.

Hex = 6 and Decimal = 10, so it is called hexadecimal.

Four bits is called a nibble (sometimes spelled nybble). A nibble is one hexadecimal digit, and is written using a symbol 0-9 or A-F.

Two nibbles is a byte (8 bits). Most computer operations use the byte, or a multiple of the byte (16 bits, 24, 32, 64, etc.).

Hexadecimal makes it easier to write these large binary numbers.
To avoid confusion with decimal, octal or other numbering systems, hexadecimal numbers are sometimes written with a “h” after the number. For example, 63h means 63 hexadecimal. Software developers quite often use 0x before the number (0x63).

How to Convert from Decimal to Hexadecimal: WikiHow