Sit at a desk or a table. Push your hand against the side of the desk. What is happening?

Your hand is pushing the desk and the desk is pushing on your hand. If the desk was not pushing on your hand your hand would go through the desk.

Newton's third law says for every action (your hand pushing) there is an equal and opposite reaction (the desk pushing back on your hand).

Let's take another example:

Kick a soccer ball. The action is your foot pushing against the ball and the equal and opposite reaction is the ball pushing back on your foot. If the ball is heavy the reaction may even make your foot hurt.