Newtons third law state that every force has an equal and opposite reaction.
This means that if object A exerts a force on an object B, as a result, Object B would exert an equal force in the opposite direction on Object A.
A force, is a push or pull of an object when it interacts with another object. For example, a person sitting on a chair would be exerting a downward force (his weight) and an equal opposite upward force of the chair.