Every action has an equal and opposite reaction.
To decelerate the aircraft, the forward thrust of the aircraft which propels the aircraft forward is reversed by a Thrust Reverser. This is accomplished by redirecting all the air ( in small engines ) or partially ( in larger engines ) to the front.
Due to the flow of air to the front end of the aircraft, a reaction force acts in the opposite direction to the motion of the aircraft. This aids in the deceleration of the aircraft.