Hex Codes or hexadecimals are alphanumeric numbers in which each digit is either a number from 0-9 or a letter from A-F. F1A200, FFF, A11 are all hex codes.
Each digit has 16 different possibilities: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F. Due to this arrangement, a 2 digit hex code can repesent 256 different numbers from 0-255. Notice that 16x16 = 256. 256 is also the number of possible arrangements of 8 bits (or 1 byte). More importantly, it allows binary numbers to be written in compressed form and reduces the number of mistakes that could occur when sharing hand written binary information.
So each byte of information can be written as a 2 digit hex string as follows: 11111111 -> 1111 1111 -> 16 16 -> F F -> FF
Hex strings can be used where humans need to read or write binary information. For example, memory addresses in computer RAM can be either represented as binary, decimal or hex. Memory address location 16777215 can be written as 16777215, 111111111111111111111111, (255,255,255) or simply FFFFFF.
Hex codes are well known to people working with html and css. Web colors in computers use hex code representation as well. Each color has Red(R), Green(G) and Blue(B) components. Degree of each of these components present in the color can range from 0-255. Therefore, each component uses a 2 digit hex code representation and a complete html color code uses a 6-digit hex code (3 bytes). 6-digit hex code can be used to represent over 16.7 million different colors (16x16x16x16x16x16 = 16,777,216)