If you happen to be working on a novel, it's good to know the key events before you even start to write.
In a screenplay, being aware of the direction of each scene is even more important as the medium tends to be more limited (especially in length).
Chapter 4: Gail discovers emails in Steve's mailbox that are vague, but still raise suspicion.
She writes to one of the senders, pretending to be Steve, and sets up a date.
Chapter 5: She goes to the restaurant to meet the email sender and sees the woman sitting on her own, checking her watch etc.
Gail picks a table next to her, but does not speak, just observes. "Chapter 7: Gail follows Steve and sees him meet the woman at a café. Beginning, story, conflict, plot,...which comes first.
A blueprint is an outline of your piece - be it a short story, novel, or screenplay.
It can be as general or as specific as you like; the idea is simply to get down in an organized fashion your plan for the piece.
You've done your brainstorming, and now it's time to put pen to paper (or fingers to the keyboard).
In a sense, writing a story is like building a house - it goes much faster if you draw up a blueprint first.