How it works

Mark needs to send personal text to Sarah, and wants that only Sarah can read it.
Mark writes here in Input field the text.
A password is automatically created. Or Mark and Sarah can estabilish a password for every communications over a secure channel (by voice, for example).
Mark can send the encrypted field OR the sharing url field over an insecure channel (for example a chat, mail, Skype, Facebook etc) where Sarah can read it.
Sarah opens the sharing url (or opens this page and pastes the encrypted text), enters the password and can read the original message in the decrypted field.

Note:MyWhisper.net and NOBODY in the network can technically see neither the original text nor the password, and not even the encrypted text if sharing url version isn't used, because all stuffs are only in Mark and Sarah browsers and all encryption/decryption is performed by their browsers with javascript.