Data is called plaintext before it is encrypted or after it is decrypted. Unencrypted information is plaintext and is input to the cipher to create ciphertext. Ciphertext cannot be read without decrypting it first which requires some kind of digital key.
If plaintext contains sensitive data, it must be handled carefully to prevent disclosure to unauthorized people. If plaintext and an encoded message both fall into the hands of unauthorized people, it could be used by an attacker to learn aboout the encryption mechanism used to encode the plaintext and could allow an attacker to get the keys used to encrypt the message. This coould allow an attacker to decrypt future messages sent using the same keys.