The concept of ‘secondary reinforcer’ comes from the famous work of B F Skinner (1938).
He found that rats experiencing positive reward – press lever, get food pellet – continued to press the lever even after the food dispenser was empty, providing it could hear the food dispenser click.
Skinner reasoned the rat had learned an association between the food pellets and the click of the food dispenser, until in the end the sound of the click became a reward in itself.
Thus, Skinner termed the food pellets the primary reinforcer and the click a secondary reinforcer. Effectively the association of the secondary reinforcer with the primary reinforcer is Classical Conditioning.