The one page synopsis must have:

The event which starts the story

The decisions and actions which the characters make to achieve a goal

The obstacles they encounter and how they overcome them

Whether they win or lose or achieve their goals

And the end of the films and where we leave the characters

You put this into your one page synopsis so when you are selling to directors or film makey people they want to make your film and give you money so you can make it. This gets them interested in your idea and choose yours over other films.