Camera projection is when you take a 2D image and give it some 3D dimensions… on that 2D image you can do animations, mappings and many other cool stuff. It has so many imitations but it has some AMAZING capabilities. The video below is an example of camera projection, but it is a VERY advanced example of it.
Mainly, they take pictures of the actors while they are in action with the background. Then they take a clear picture without the actors for the background, then they do some camera projection to the background and then they map the actors into 3D models in order to give them a 3Dish feeling when the camera rotates them. With some color correction and a good montage of the resulted footage you can see the result in the video below.
I hope you enjoy it! :)