This success message tells you PrismCipher has just encrypted the text you selected (not necessarily the entire message).
Note: HTML content can be quite complex. The internal markup can contain much more than just the text and images you may see. When PrismCipher cannot successfully encrypt just your selection, it will encrypt the entire message. It may also start with your selection and encrypt all the way to the end of the message. It does this to ensure the integrity of the encrypted contents and to ensure the message can be successfully decrypted.