Basically, this text appears on your monitor (the radioactive screen) when you try to navigate in your history, and a page has expired from cache. So, your computer can't really remember what you saw on this page, so it needs to request it back from the web server. However, the page beforehand submitted some POSTDATA to the web server, so if you were to access it again, very bad things might happen (such as resubmitting an online payment).