Depth of Field (DOF) is the simulation of lens focus on the subject of the scene. This effect emphasizes on the subject by blurring everything ahead and behind the subject. In the blurred region, all the tiny highlights expand to exhibit shapes called Bokeh.
Blender provides a Defocus node to simulate DOF. The following screenshot depicts the result of the DOF application as a postprocess in Blender Compositor. From the following screenshot, it can be observed that the emphasis is on the red block, blurring the other blocks and simulating the focus effect. This node requires an image and corresponding Z Depth inputs.
The Bokeh type can be set to modify the shape of the Bokeh, emulating a circle, triangle, square, pentagon, hexagon, heptagon, or octagon. Beyond octagon, there will not be any significant difference, as compared to the circle mode. The following screenshot shows different Bokeh shapes on the blue block:
