This is a Chrome extension that lets you download your Game DVR clips from your profile page on account.xbox.com
Your clips are only synchronised and uploaded with an active Xbox Live Gold subscription, but once you run out of Gold and you want to upload the clip to OneDrive to access it from - for example - a computer, it will ask you to buy a Gold subscription. All this script does is redirect you to the MP4 URL which will allow you to simply right-click on the video, and save it.