All videos will buffer for about 5 to 10 seconds before they begin to play. This is so the video can begin to "buffer" (download) before it actually plays. This prevents the video from trying to play before enough of the video file has loaded to enable playback.
Buffer progress is displayed by the black progress bar as illustrated below. Under idea circumstances, the buffer progress bar will always be ahead of the the playback head (blue button). If you find your connection speed is slow, simply pause the video until the buffer progress bar is ahead of the playback head. Once the video loads fully, (when the buffer bar has fully reached it's end) you may use the playback head (blue button) to advance the video forward and backward.