What is frequency analysis？

Frequency analysis is a technique used in cryptography to analyze the frequency of letters or symbols in a message or ciphertext. By examining the frequency of occurrence of certain letters or symbols, it is possible to uncover patterns or clues that can help in decoding the message. This technique is based on the fact that certain letters or symbols appear more frequently than others in a given language, such as the letter "e" in English. Frequency analysis can be used to break simple substitution ciphers and is often used in cryptanalysis.