A "dead" or "broken" external link is a link to somebody else's website, which don't work any more. There are systems, which automatically checks external links (normally on a subscription basis). HTMLCleanUp permits you to do it manually, by creating a list of all external links, and then you can check them yourself, simply by clicking them in the build-in MS Explorer 4 browser.
If MS Explorer 4 or later is not installed on the PC, the browser function is blocked, but the rest of HTMLCleanUp will work. The external links list can then be saved as an HTML file to be viewed with your normal browser.