When an object that is moving slows down, it means that its velocity is decreasing. This can be caused by either a decrease in its speed or a change in its direction. In either case, there must be a force acting on the object that is opposite to the direction of its motion. This force is commonly referred to as resistance or friction.

For example, if a car is moving forward and it starts to brake, the force of friction between the tires and the road will act in the opposite direction to the car's motion, causing it to slow down. Similarly, if a ball is thrown into the air and it starts to fall, the force of gravity will act in the opposite direction to the ball's motion, causing it to slow down and eventually come to a stop.