To separate a subject from a background, you can either:

1. shoot in front of a greenscreen/bluescreen, then key out the background in postproduction

You can also key out a background that is much darker or brighter than your subject, but it usually takes a little more work, because often your subject will have pixels that are darker and/or lighter than the background, so there there will be holes in your subject that must be “patched” with a garbage matte. If your subject is not moving much, this is much easier than if you have to do a lot of keyframing on …