The ciphertext mnemonic is the mnemonic encrypted with the wallet password. When backing up your wallet, you can choose to back up the ciphertext mnemonic QR code.
Because the ciphertext mnemonic is an encrypted mnemonic, even if it is stolen, the hacker needs to crack the password to steal the assets. The ciphertext mnemonic is encrypted with a Scrypt-based hash password, and uses the same encoding format as Keystore.
Remember the wallet password while backing up the ciphertext mnemonic. Forgetting the password will invalidate the backup.