This is a tool to share secrets via a one-time only link. It works by encrypting the secret on your end, sending the encrypted secret to the server, and then providing a link that allows the secret to be retrieved and unencrypted. The encryption key is never sent to the server, and you can check that that's true in the javascript source code.

Once the encrypted secret has been retrieved, or after a pre-defined period of time (currently one day), the secret is deleted and is no longer available.

Keep in mind that once someone else has the secret, nothing prevents them using it however they want, including distributing it insecurely to as many people as they like.