When the notebook is sitting on the table, what force prevents it from falling into the space below the table? If you don't know the name of this force, just describe it. What force keeps the notebook from floating in the air?

This force is called the reaction force or also known as the normal force. This is a concept from Newton's Third Law of Motion, which states that for every action, there is an equal and opposite force. This force is what keeps the notebook on place on top of the table. It is the force exerted by the table to the book.