It's probably some web site that uses it triggering the request you download something to your PC. You can ignore it safely. If you do want to download something then I think it better to visit the apple site yourself and decide what to download.
Thanks O-G - it comes up on this site sometimes, and eBay just now. It's an annoying banner because the options are "allow" or "allow for all websites" - no "no thanks" option! - so I just close it. Damn, it's just popped up again as I type this!