Storyboards are sketches, similar to a large comic book of the film or select scenes produced beforehand to help directors, cinematographers, and advertising clients visualize the scenes and find potential problems before they occur— in short, a complete blueprint for the entire production to work from.
Storyboards get everyone on the same page while giving you the flexibility to explore options and make changes for a fraction of the cost of doing so on set, resulting in a cheaper and more efficient production while reducing the need for reshoots.