Usually, black bars at the top and bottom of a video indicate that the screen is not the right-size format to fit the video. The display fills the screen width, and the black area is where there is no video signal. You can convert videos to fill in this space, but the process distorts the whole image. If you keep a wide-screen video format the same, the black bars will fit whenever it is shown on a standard-size screen. But if you want to add the bars so that they show even on wide screens, you can do so by converting the video format.
