This is an old trick, so I understand why people probably haven't seen it these days. But you have a page with two frames: the submission frame (i.e. input form, usually on the bottom of the page) is reloaded on submit, while the chat log (usually the top frame) loads infinitely.

Just to be further specific, they're talking about embedding two separate webpages inside of one page, using "iframe" HTML elements. In the bad old days you'd probably use a bare "frame" element, but those aren't valid HTML as of HTML5.

I think they used a frameset with frames. It had the advantage that it could be sized to the browser window as a percentage and both frames could be refreshed/loaded independently without affecting the other.