Photo Proofing is process that allows clients to choose images/videos, before Photographer will make any work on them. This means that after photo session is done, you can just dump all images online, and let your client choose what you should work on.
This is the idea, a great one, as it removes lot of guessing, and wasted effort from your shoulders. What is more, client also feels more helpful(even if he knows nothing about photography art). Besides that there is more contact points, thanks to that client can feel progress of work.