A method for writing out long binary numbers is to break the bits up into 4 bit groups which is quater of the length if it was written out in full fro example: 0101 0011 1001 0001 which equals 5931. A problem is that each group of 4 bits can go up to 1111 which is 15. Becasue the digits only go up to 9 we need to use letters as well. The letters represent the numbers 11 to 16 as shown below:
A = 11 = 1010
B = 12 = 1011
C = 13 = 1100
D = 14 = 1101
E = 15 = 1110
F = 16 = 1111
We need this in computers because hexadecimal is a good way to write binary data in a condensed form, which is far more convenient to write out and represent binary data to humans because of the decrease in size. Hexadecimal is used in html, the hexadecimal is used for the colour codes by shrinking 24-bit colours down to 6 digit hexadecmial values.