Encoder requires TXT file and a PNG image. After submission API performs the steganography computation, and download page displays the key and download button for your steganographed image with embedded message from the TXT file.
The key is important, remeber it or save it because without that you hardly reverse the message from image. If the key won't match, API throws error. To extract the message paste the key, load the PNG steganographed image and press "extract" button, message will appear on the page. For more information about Steganography itself and Least Significant Bits, visit this blog post.