A rail fence cipher is a simple form of encryption that rearranges the letters of a message to create a coded version. It works by writing the message in a zigzag pattern across multiple lines, resembling a fence. For example, the phrase "HELLO WORLD" can be written in two lines as "HLOOL" and "EL WRD," which are then combined to form the ciphertext. To decode a rail fence cipher, the recipient must know the number of lines used in the original encoding. By reconstructing the zigzag pattern, they can read the original message. This method is easy to implement but offers limited security against more advanced cryptographic techniques.