what is stage management ?

Although a somewhat fluid line of work, in essence the stage management team (which can consist of a production stage manager, several assistant stage managers, and any number of production assistants) is responsible for organizing the production, communicating across different disciplines (e.g., between the director and the backstage crew, or the actors and production management), and keeping everything running smoothly. This refers not only to seamless management of the technical aspects of a production, but of the human aspects as well.

which means stage managers are assistants of the director, handling matters at the backstage and making decisions if anything goes wrong when the show is on (becuz the director is not gonna be at backstage at that time)