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 substitution cipher, the letter 'A' might be replaced with 'D', 'B' with 'E', and so on. These ciphers are easy to create and understand, making them popular for basic encryption. However, they can be vulnerable to frequency analysis, where the most common letters in the ciphertext are matched to the most common letters in the language, potentially revealing the original message.