Hello anon! The thought is that the beginning is a mirror of the end. When you know what your script is about, what will happen to your protagonist and the way in which they will succeed or fail, you can devise meaningful scenework to set all of that up while leaving out any vestigial false starts that dilute your purpose in the first act.
The first act has a lot of work to do and has to look good doing it. Knowing the end gives you all the puzzle pieces to fit the first act together so that nothing is left out and nothing extra is left in.
Or at least that is my understanding of the quote.