Smalltalk.page is a end-to-end and all client-side encrypted messaging service. It is perfect to be used to exchange confidential or secret information to another person or a group of people with a pre-shared password, which is the only key able to decrypt the message.
A person having the URL you sent but not having the password cannot decrypt the message. This implies even your ISP cannot decrypt it. Since we don't keep your password, even we cannot decrypt your message.
Since this is all JavaScript, all source code can be found by right-click and 'View Source'. The commit history can be found on this page as well.
Warning: there are still some weaknesses in the communication. For example, a browser extension with the ability to read the full content of a page can still read your information. Turn off those extensions if you have concerns.