Newton's Third Law of Motion states that for every action, there is an equal and opposite reaction. This means that whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object.
When you sit in a chair, your body exerts a downward force on the chair due to gravity. According to Newton's Third Law, the chair will exert an equal and opposite force upward on your body, which is why you don't fall through the chair.
Another example is the propulsion of a rocket. The rocket expels gas at high speeds in one direction (action), and as a result, the rocket moves in the opposite direction (reaction) with an equal force.