A substitution cipher is a method of encoding messages by replacing each letter in the plaintext with another letter. This can be done using a fixed system, where each letter is consistently replaced by another letter throughout the message. For example, in a simple Caesar cipher, each letter might be shifted by a certain number of places in the alphabet. Substitution ciphers can vary in complexity, from simple letter-to-letter replacements to more advanced systems that use symbols or numbers. While they can provide a basic level of security, they are relatively easy to break with techniques like frequency analysis, which examines the frequency of letters in the encoded message.