You earn points for easy braking, acceleration, and cornering, as well as steady speed. You know you've earned points because the screen turns green, accompanied by a gentle sound.
However, if you brake or corner hard, accelerate too quickly, or fail to maintain a steady speed, you lose points. In those situations, the screen turns red and you hear a warning sound.
At the end of each trip, you can see your total score in the upper left corner of the overview. If you tap "All Trips", you can see the score for each trip you have recorded.