The command “back to one” is one of the most common ones used on set. After a film take, the 1st AD (Assistant Director) will tell actors and background actors as well as crew members to go “back to one”.
In other words, actors and crew have to go back to the original location of the top of the scene. When everyone is set and able to go again, the 1st AD says “Rolling”, “Sound Speed”, during the pandemic “Masks off” ( for actors and extras), “Background” and “Action”. Actors and Background start their action till the director or 1st AD yells “Cut!”. Synonyms are “back to first” or “reset”
Here an example from “The Making of ET”.
