The rail fence cipher is a simple form of transposition cipher used in cryptography. It encrypts a message by writing it in a zigzag pattern across multiple lines, resembling a fence. The message is then read off line by line to create the ciphertext. For example, the word "HELLO" might be arranged in a two-rail pattern, resulting in a different sequence of letters. To decrypt the message, the recipient must know the number of rails used and reconstruct the zigzag pattern. This method is easy to implement but offers limited security, making it suitable for basic encryption tasks rather than sensitive information.