A contact sheet is a method used by photographers to easily display the work from a photo-shoot and scrutinize the images, making a fine selection of final images from the entirety of the photo-shoot.
They are useful as they allow a photographer to see all of the images at once on a single sheet and notate any changes that they may want to make to the images as well as entirely disregarding images if they are beyond editing or are simply not composed to their standards. Displaying any errors also allows the photographer to progress their work and learn from the mistakes more easily.