Frequency analysis can be performed on polyalphabetic ciphers which makes it weaker than monoalphabetic ciphers
Answers
Substitution ciphers are normally most common form of cipher.
They work by changing each letter of the plaintext with another letter/random sign.
Monoalphabetic substitution cipher also called as simple substitution cipher depends on fixed replacement framework.
That is a substitution which is fixed for each letter of alphabet.
If "a" is encrypted to "R" then each time we see letter "a" in plaintext and hence we replace it with letter "R" in ciphertext.
Simple demonstration is where each letter is encrypted as next letter in alphabet: "a simple message" becomes "B TJNQMF NFTTBHF".
Normally when replacing a simple substitution manually it becomes easier to generate the ciphertext alphabet first and then encrypt by relating this to plaintext alphabet.
Table underneath shows how one could lay them out for this demo.
Varieties of monoalphabetic substitution ciphers infinitely many as every letter can be encrypted to any sign not just another letter.
Record of simple substitution ciphers can be traced back to very earliest civisilisations and for long time they were more plenty for a reason for which they required.