Think about the shape of an old-school standard definition television (SD, 4:3, almost square) to a modern HDTV (HD, 16:9 widescreen, rectangular). Most television and film today is shot for the modern, widescreen look. But older footage (as well as anything new that is emulating an older style) is shot in the old-school aspect ratio.
The problem is when you put together a reel from footage with different aspect ratios. It is crucial that your editor present each piece of footage in the aspect ratio it was shot in - otherwise everything will look distorted (If you present SD footage in an HD aspect ratio, everyone looks stretched out - heavier. And if you present HD footage in and SD ratio, everyone looks warped and too thin.) On a modern, widescreen reel, this means you'll have black bars on the side for any old fashioned, 4:3 footage (this is known as pillar boxing), and you will not have any black bars on your HD television footage, since it is broadcast in the same 16:9 ratio. However, some bigger budget movies are shot in a "wider" ratio, so they may display black bars at the top and bottom of your footage in some cases (this is generally referred to as letter boxing).
The exception to this is reels that are output for actors access - which still uses a 4:3 aspect ratio. In that case, pretty much everything ail have the "black bars".
We're putting together a video which will explain this visually. In the meantime, here's a more in-depth article about aspect ratio.
It's a personal pet peeve of ours when we see reels that have been output by professional editors that do not display their clients' footage correctly. There's simply no excuse.
At ActorFusion, we take pride in doing it correctly.