That's normal. The aspect ratio of a movie isn't necessarily going to be the same as your TV. Your TV is probably 16:9 (1.78:1). Movies don't all have the same aspect ratio, and they can be different from that. If you have the box to the disc, it probably says somewhere on the box what the aspect ratio for the movie is. If you let us know which movie, we can also look it up for you.
Your TV (and maybe disc player) might have options that will modify the picture to fill the entire screen if you would prefer to not have the black bars. It would stretch things out, though, so the objects on the screen wouldn't look exactly as they did in the original movie.