3D Projection is a technique where you use a light to shine through a piece of footage and project the image onto any 3D layers in your scene. This works much like an old school slide show projector. If you are clever about setting up your 3D layers in After Effects, this allows you to somewhat re-construct the geometry of the original scene.
Once you have this 3D projection set up, you can add a camera into the scene and move it around and, within limits, it will appear to be moving around in the actual 3D scene
This effect is a bit more advanced so I will assume that you have a basic understanding of how to use After Effects and have worked with 3D layers before. Let's get started!