Whether you’re acting, standing-in, or modeling for print, your “mark” is the exact spot your feet (and body) must be for your performance. So when you are standing “on your mark,” you are in the place the director, casting director or other production crew members have instructed you to stand. Your “mark” is where your feet must be for your body to be in the proper lighting and camera shot.
One more thing – stay on your mark. Don’t lean or wander… To stand on your mark is to stay on your mark. You don't have to be a statue, but that is your space to stay contained, so stay put, in-focus, and on-mic.
Courtesy: https://tophollywoodactingcoach.com/2012/07/whats-stand-on-your-mark-mean/
