Masked Forwarding is a different kind of forwarding. If you use HTTP forwarding above, when your browser gets the 301 or 302 response with the new address for the webpage, then you’ll see the URL change in your browser’s address bar.
Masked forwarding is “Masked” precisely because the address in the address bar remains unchanged.
This creates an HTML page that displays the page being forwarded to in an invisible “frame.” A frame is an HTML element on a webpage that acts like a kind of window to another webpage. It points to a different webpage and displays the contents of that page within the frame. This can be useful for web design, and it can also be used to create a webpage that consists solely of such a frame, so that the result appears the same as the forwarding methods described above, but without the address changing in the address bar.
It would appear as though the content being forwarded to was just the content of the page navigated to, hence the forwarding is “Masked.”