Monoalphabetic cipher uses a permutation of itself as a key and then substitutes the plaintext using this key.
It gives us n! possibilities of keys. Ie. it is kind of resistant to brute force. Nevertheless, using the statistics and properties of natural language we can analyze the frequency of letters’ occurrence and map corresponding alphabet-key pairs. Simply said the plaintext frequencies translate to ciphertext frequencies. The longer the message the more accurate results we can get for certain letters and then deduct the rest.
The most often letters in English are “e” and “t”, the least “z” and “q”.