The Rail Fence Cipher is a simple transposition cipher used for encrypting messages. It works by writing the message in a zigzag pattern across multiple "rails" or lines, then reading off each line to create the ciphertext. For example, with three rails, the message "HELLO WORLD" would be arranged in a zigzag and then read line by line. This cipher is easy to implement and understand, making it a popular choice for beginners in cryptography. However, it is not very secure by modern standards, as patterns can be easily detected and decoded. It serves as a good introduction to the concepts of encryption and decryption.