If you want to learn how to write and sell a script, you must also learn how to write a synopsis.

A movie is based on a screenplay, but that screenplay is based on a story. And a story in its purest form can be found in a movie synopsis.

Whether you plan on writing a film synopsis for a script that's done and dusted or is yet to be written, being able to refine a story down to 400 or so words is an invaluable skill to acquire.