Motion blur blurs objects based on its motion. The system works by way of a full screen velocity map that is created at a reduced resolution and objects are blurred based on their contribution to this map. The image below shows a visualization of the velocity map available in-editor or while your game is running.
This visualization mode is enabled with the
ShowFlag.VisualizeMotionBlur 1 console command, or by way of the Motion Blur Visualizer found in the viewport under Show > Visualize > Motion Blur.