The description of WebM for IE9 - Play With New Friends
WebM for IE9 is an application created by Google to enable support for playing WebM videos in Internet Explorer.
IE9 doesn't support WebM videos when you install it, so you'll need this add-on to view WebM videos as usual.
WebM is nowadays one of the most promising video formats and you should be prepared to watch them in your browser.
Remember that if you use Firefox, Chrome or Opera you won't have to install anything because they support this format without the help of any add-on.