Objects in contact exert forces on one another. Example: a book on a table exerts a downward force on the table, and the table exerts an equal and opposite force up on the book---the normal force N.

The normal force is measured by a scale placed between the objects in contact. Your "apparent weight" (how heavy you feel) equals the normal force you are experiencing.

Objects lose contact with one another when the normal force goes to zero---think catching some air!