What are encryption messages?

Encrypting a message protects the privacy of the message by converting it from plain, readable text into cipher (scrambled) text.

Only the recipient who has the private key (private key: The secret key kept on the sender's computer that the sender uses to digitally sign messages to recipients and to decrypt (unlock) messages from recipients. Private keys should be password protected.) that matches the public key you used to encrypt the message can decipher the message. This is a separate process from digitally signing a message.

Source: http://office.microsoft.com/en-us/outlook/HP063699521033.aspx