Compositing is the process of blending together a variety of different 2D and 3D elements into a final image or image sequence. It is very rare to render a complete scene or shot in one go; instead, current practice has elements rendered out as necessary from the 3D content application, manipulated as necessary in the compositing evironment, and then final output to the desired format.
You will find that renderings occur much more quickly when rendering specific elements, as opposed to the whole scene. Similar benefits can be enjoyed by rendering elements in Cinema 4D, and then re-using elements again for the same shot. For example, you can render a separate background pass, and then add the background in as a 2D element for the final rendering, which saves the render hit from the background for all of those frames. And that time saved is likely to be significant. Further, you can use projection mapping on simple geometry along with rendered imagery from a higher resolution scene, and enjoy the benefits of much faster, yet dimensional, backgrounds, and generate your complete shots that way.
Whatever method you choose, the following articles cover definitions for typical compositing terms you are likely to encounter.