A contact sheet is a positive print of all the negative images from one film, made by a contact printing process so that all the images are the same size as the negative. A contact sheet is a useful way of seeing which are the best images on a film so you can decide which ones to make enlargements from.
Contact sheets a date back to the days of film. When developed negatives are laid directly on top of photographic printing paper, exposed for the appropriate duration, and fixed. They aren’t ‘proper’ prints per se, but a provide the photographer/editor/client way of looking at all of the images in a sequence at once.
Seeing all the pictures you have taken at once gives a opportunity to judge each image, compare with similar shots from same shoot, but bare in mind some images look a lot better when full size.
They are a useful tool for selection, critic and more.