Occlusion culling is a technique that is broadly used in games, especially 3D ones. Since drawing is very expensive and has to be done many times, every optimization counts. The obvious optimization is to not draw the parts that are not going to be seen (for example hidden by something else). If we can draw each pixel on the screen only once, we are saving a lot of processing time.
The fact of drawing each pixel more than once is called overdraw. Having a high overdraw is one of the factors that impact performance the most.
In the case of 3D, drawing is especially expensive, and occlusion culling is something that most engines do automatically to a certain extent.
Occlusion culling optimizes the drawing time by not drawing what ...