The paper describes the secondhand market for used cars. Some cars are in good working order — these are referred to as jewels; Some have hidden defects — these are called lemons. Yet because buyers don't know which cars are the lemons—in this asymmetric market—the market price of even a good car decreases. Thus, sellers of good cars are less economically inclined to sell their cars, and a rational market will only be filled with bad cars.
The term lemon, meaning a defective (typically used) car, entered the language as a result of this paper.