This is a commonly used term in the motion picture and TV industry but if you have no clue what it means it separates the experienced and the novice in seconds.
Back to one means back to the beginning or starting point in the scene.
This is a vital time because production wants to quickly go again. If the scene is a big war scene this could be timely for everyone to get back to one.
If the scene is 40 actors walking down a high school hallway and the director yells cut and the 2nd AD yells "CUT" and then " OK BACK TO ONE EVERYONE". Do yourself and everyone on the set a favor and get your butt as quickly and safely as possible back to YOUR starting point. Don't wait for anyone just get back as soon as possible and people WILL notice. They will also notice the people nonchalantly strolling back. But you will get the call for more work because you helped production save money.
Essentially the quicker you get back to the starting point the quicker they can shoot the scene again.
If everyone hustles back they can get going again quickly.
