Computer data is stored as a combination of ones and zeros. Each one or zero is called a bit. A combination of eight bits is called a byte. A bit can have 2 combinations whereas a byte can have 256 combinations and is commonly used in text documents.
Each character in a text document is one byte long and so can be any number from 0 to 255 for a total of 256 characters. This would include all the characters in the alphabet in both lower and upper case as well as numbers, punctuation and some special characters. Have a look at your keyboard. Each character has a corresponding number. For example the letter “a” would be the number 97 or using bits it is 01000110.
The eight ones and zeros correspond to eight combinations of the numbers 1,2,4,8,16,32,64,128. With any combination of these eight numbers you can get any number from 0 to 255.
For example the byte, 10000000 is the number one. 01000000 is the number two. 00100000 is the number four.
If you do a combination, you just add the numbers up so 11000000 is the number three being 1 + 2. 10000001 is the number 129 being 1 + 128. 11111111 is 1+2+4+8+16+32+64+128 = 255.