The event.

The event is what the 2nd AD writes in the call sheet for each scene.        

For example:

‘Josh nervously divulges his plan to the audience.’
‘Dr. Hertzberg provides another breakthrough.’
‘Lucy gets educated in finance.’
‘Lucy and Josh find the picture in a drawer.’

These descriptions consist of a proper noun – the character; a verb – what he does; and the complement noun or subject – the recipient of the action. 

The event describes the central action of the scene. Is the main reason the writer wrote the scene. Sometimes it is something new for the audience. 

‘Josh nervously divulges his plan to the audience.’

Josh may never have revealed his plan before so that is new to the audience and is a plot point. 

‘Lucy and Josh find the picture in a drawer.’

Maybe Lucy and Josh’s search for the picture was a McGuffin and the finding of it a turning point in the story.

The event always tells you in simple terms the crux of the scene.

Knowing the event helps you to play sharper.