Compositing

Compositing is an essential part of visual effects, which are everywhere in the entertainment industry today—in feature films, television commercials, and even many TV shows. 

It involves taking the render passes from the production stage and combining them together or mixing in additional images or adjustments layers, to create a more cohesive composition.

The process can be as simple as putting two layers together, or as complex as matching hundreds of layers and adjusting their properties such as depth of field and color. This is where the final look of animation really comes together.