Real encryption
This is the same kind of encryption banks and messaging apps use: AES-256 in GCM mode, done by your browser's built-in Web Crypto. Your password is stretched into a 256-bit key with PBKDF2 (600,000 rounds of SHA-256), which makes every guess slow for anyone trying passwords one after another. A fresh random salt and starting value are used every time, so the same message and password never give the same result twice.
Your password never leaves the page
Everything happens on your device. The message and the password are not sent anywhere and not saved, so there is no copy on a server for anyone to find. Send the encrypted text however you like, and give the password to the other person a different way, such as in person or by phone.
Decrypting
Pick Decrypt, paste the text that starts WT1., type the password and press Decrypt. Switching to Decrypt right after encrypting carries the result across, so you can check it works. A wrong password, or text that has been changed by even one character, is refused rather than turned into nonsense, because GCM checks the text has not been tampered with.
A lost password cannot be recovered
There is no reset and no back door: without the password the message is gone for good, and we could not help even if asked, because we never see it. Use a password you will remember, or keep it in a password manager. A long one is far harder to guess than a short one; the Password Generator can make one.
Questions
Is this the same as the secret code translator?
No. The Secret Code Translator is for puzzles, and its codes can be cracked by hand. This page uses modern encryption that cannot be broken without the password.
What does WT1. at the start mean?
It marks the format, version 1, so the page knows how to read it back. After it comes the salt, the starting value and the encrypted message, all in Base64. Copy the whole thing, dot included.
Can I decrypt it somewhere else?
Yes, with any AES-GCM tool that lets you set the details: PBKDF2 with SHA-256 and 600,000 rounds, a 16-byte salt, then a 12-byte starting value, then the encrypted bytes with their 16-byte tag at the end.
How strong should the password be?
Treat it like any important password: at least 12 characters, and not used anywhere else. The encryption itself is unbreakable in practice, so a guessable password is the only weak point.