This page uses public-key cryptography to encrypt text in realtime.
Public-key cryptography is a form of asymmetric cryptography - this means that the key, or password, for encrypting data is not the same as the one for decrypting it.
This allows a public key to be included with this page, which can be used to encrypt data, without compromising the security of your message to people who load the same page later.
Without the other half of the key - the private key - the data is gibberish, even if you have the public key that was originally used to encrypt it in the first place. Private keys are carefully held and secured by individuals or businesses, as they are used to decrypt data which was encrypted by their matching public key.
Since I am the only person who has my private key, I will be the only person able to decrypt messages that you encrypt using this page.
For this reason, you can safely send these messages over insecure channels, like email, without worrying about the message being intercepted, or the email being maliciously accessed later. The attacker might get their hands on the encrypted data, but will be unable to turn it into anything meaningful.