Unlike Caesar, each letter is replaced by a different letter according to an arbitrary mapping (not a simple shift). We solve it by comparing the letter frequency of the ciphertext to the known letter frequency in English, and guessing short words ( the , and , a ... ).
Source: [Dev.to](https://dev.to/ctfdojo/picoctf-substitution0-writeup-break-a-substitution-cipher-with-frequency-analysis-44l8)