What is a shot list?
A shot list is a list that gives filmmakers a general idea of every shot they foresee being in the project or movie. A list of the movie edited inside a filmmaker's head.
To create a shot list, use the script and read through the movie scene by scene and write down every shot (medium close-up of x, long shot displaying y and z, etc.) that the scene should contain. In addition to listing the type of shot, a shot list can include whether the shot is tracking, a dolly, panning, or any zooms used and even the type of lenses -- essentially anything imaginable by the filmmaker.
Some other terminology used to clarify shots in a shot list is stating if the shot is dirty or clean. A dirty shot means there is an object or subject in the foreground, such as an over the shoulder shot, and a clean shot, such as a close-up, means the opposite.
Why is it so important?
Shot lists are important because they allow film crews to work quickly and efficiently. You are essentially looking into the future, planning tech around how to get to the future, and this will produce results.