You've been sent an e-mail, and it's critical the contents are safe from prying eyes. In the case of Gmail, and a handful of other popular Web-mail providers, your e-mail could be in a dozen different servers (albeit encrypted), or even be analyzed to try to sell you contextual ads.
The creators of Lockbin would like to help you avoid such security calamities with their closed system that will take any message and send it to someone in a highly secured manner. How secure? You can't even open it unless your recipient happens to have the "secret word" which acts as the encryption code. When it's time to actually decrypt a message, the entire process is done within the recipient's browser window instead of Lockbin's servers, meaning if it gets into the wrong hands the message's security is not entirely compromised.
Lockbin's creators note that "nothing is perfect, and neither is this," but I must admit, any system that requires a captcha to even get to the point of writing a message feels like going through airport security.