ASCII stands for American Standard Code for Information Interchange. Frankly speaking, it is a table for conversion. Computer transforms binary code to a human readable symbol.
Characters in ASCII encoding include uppercase and lowercase letters A through Z, numerals 0 through 9 and basic punctuation symbols. It also uses some non-printing control characters as well.
ASCII is usually used with binary or decimal codes. So, if you see binary code made from 1 byte (which is equal to 8 bits or simply put - 8 ones and zeros) then there is a chance that binary code could be changed to a regular symbol trough ASCII table.
Here is the conversion table:
As you can see the regular ASCII table has 128 different symbols. Each symbol has a value of decimal number which could be converted to any other format including binary. So if you change binary code to decimal number you can then use this table and get a letter.
For example, letter A is equal to 65. 65 is equal to 1000001. So A=65=1000001. You can transform one to another easily.
ASCII is widely used in harder puzzle games.