Newton's states that when one object exerts a force on a second object, the second object always exerts a force back on the first object that's equal in magnitude and opposite in direction.

Explanation:

Newton's third law states that every action force has a equal and opposite reaction force.

Example 1:

Consider yourself hitting a wall with your hand,you are exerting a force on the wall but still you feel hurt because the wall exerts same force on your hand in opposite reaction.

Example 2:

Consider yourself standing on ground.You are exerting force on ground.But how are you able to stand on ground even if your mass is forcing you to go down?The answer is normal reaction force.The ground exerts a force on your feet which makes you to stand still.