What is the English sentence shouted at the beginning of the film?

Action, the director's special language, the oral language that the director ordered the actors to start performing at the shooting scene.

The real meaning of action:

The director doesn't use verbs such as act, start and begin when shooting. The reason is that during the performance, the actors are just expressing the film that the director has conceived. When shooting, the director will compare whether the actions performed by the actors conform to his own thoughts and feelings. If they are consistent, he will go on to the next shot. If it feels wrong, it will be repeated.

So the real meaning of action is that the director is ready to watch the actor's performance, which is why he chose to use this term instead of directly using act.

Extended data:

When the director calls for action, it is called "slap", and the black and white board is called "slap" in the circle. At this time, the camera has been turned on and recorded what was written on the slate: location, scene, number of pictures taken, etc. This shot, so that later editors can quickly find the required material. After a click, the note quickly left the machine and the actor began to perform and shoot.

References:

Baidu Encyclopedia-Action