Newton's third law of motion states that "for every action, there is an equal and opposite reaction'. Here, action and reaction are forces and are always coupled to each other, that is, if there is an action, there will always be a reaction. In simpler terms, if a body exerts a force on another body, the second body will also exert a force on the first one. Both the forces will be same in magnitude, but will act in opposite direction (from body A to body B and body B to body A). This means that a book lying on a table is exerting some downwards force on the table (action) and in response, the table is also exerting an upward force on the book (reaction) and that both these forces are equal in magnitude.
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According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton's third law of motion. Formally stated, Newton's third law is:
For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.