The ApplicationCache (AppCache) is a feature of HTML5 that aims to allow websites and Web applications to be reliably accessed while offline.
You can find out more about the AppCache in this great HTML5 Rocks article.
Unlike other online AppCache generators, this one uses actual network requests from a page instead of only scraping URLs from top-level files. This means that it includes asynchronous assets that may be requested after the page has finished loading.
The generated manifest should not be taken as-is. It's a good starting point but should be manually tweaked to suit your specific needs.