Motion blur is used to mimic the effect of a physical camera (and the human eye). Rapid movement will appear streaked over a single exposure (or frame) depending on the shutter speed. A slower shutter speed results in a longer exposure resulting in a blurrier image.
In computer animation, a lack of motion blur can result in fast moving objects appearing stepped or staggered. You can prevent this by adding motion blur.
To enable Motion Blur on a Shape you need to do two things:
Enable Motion Blur in the Composition Settings. This window can be accessed via the cog icon at the top of the Project Window. Open the window and check the Motion Blur attribute.
Enable Motion Blur on the Shape. This can be done via the Attribute Editor. Load your Shape's UI and check Motion Blur under the Shape tab.
Motion Blur - enable/disable Motion Blur.
Shutter Angle - set the angle of the shutter. The default of 180° will give a you a 'filmic look'. That is, 50% of your frame rate.
Motion Blur Centre - affects whether your blur is backward or forward of the current frame. The default of -0.25° will centre it on the frame (your motion blur will be even in both directions of travel).
Motion Blur Samples - determine how many times the motion blur is sampled. A higher number will result in a smoother blur but also comes with a performance penalty.