Click on one of the thumbnail images to play that video.
If you have too slow of an internet connection, the video playback will be jerky. In this case, click on the video pause control and wait for the buffering progress bar across the bottom of the player to finish. Now you can playback normally.
An uncommon problem is for the playback to suddenly stop before the video is over. This is usually caused by a corrupt cache of the video in your browser. Try clearing your browser's cache and reloading the page.