Some videos on the iPhone or iPod touch have horizontal or vertical black bars bordering the screen. This is because the original composition of the movie or video is too wide or narrow to fill the screen. If you want to have the video fill your entire screen, simply double-tap anywhere on the screen, and the video image will zoom in slightly and magnify the screen so that there are no black bars. Double-tap on the screen again to return the screen to its original configuration. There is also a button for this on the playback controls (which you can make visible by single-tapping the screen) in the top right corner of the screen.