If video playback is choppy, chances are that your connection is having difficulty downloading the video fast enough to play it in real time on your computer. You can see how much of the video has downloaded when looking at the progress bar. The lightest bar shows the length of the video. The darkest bar is how far along you are in the video. The bar in between those two, with a medium gray tone, displays how much of the video has loaded so far. If you pause the video and wait for that bar to get a bit farther you'll have an easier time watching without stuttering playback.
