The M9 motion coprocessor is a specialized chip designed by Apple to enhance the performance of motion-related tasks in devices like the iPhone and iPad. It collects data from various sensors, such as the accelerometer, gyroscope, and compass, allowing for improved tracking of movement and orientation. By processing this data efficiently, the M9 coprocessor enables features like fitness tracking, navigation, and augmented reality applications without heavily taxing the main processor. This results in better battery life and smoother performance for users engaging in activities that rely on motion detection.