Simplify the following text for a Grade 9 native English-speaking person.
Original: In cryptography , a Caesar cipher , also known as a Caesar 's cipher , the shift cipher , Caesar 's code or Caesar shift , is one of the simplest and most widely known encryption techniques . It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet .
Simplified: The Caesar cipher is a substitution cipher : each letter is replaced by another .