According to Newton's third law, for every force "P" that is applied on an object there is a force -P that has the same maginitud that "P" and opposite direction.
In other words "for every action there is an equal and opposite reaction"
For example, if Object D applies a force "P" on an object B, then object B will make a force -P on object D.
This force is called the reaction force.
Therefore when the swimmer pushes the wall, then the wall pushes the swimmer with a force of the same magnitude that is called the reaction force.
Well for starters, gravity keeps things down on Earth. Also, gravity applies its force on to cars to maintain stability by forcing the tires to make constant contact to the ground creating friction between the rubber and road surface allowing the car to be moved. I'm sure someone else has better ideas!
