What is Consumer surplus?

The difference between the highest price a buyer is willing to pay for a good and the price actually paid. This is the buyer's "gain": he or she obtained the good more cheaply than he or she valued it. The concept was popularised by the economist Alfred Marshall, who regarded it as a measure of the extra benefit a person derives from his or her environment. At the market level, consumer surplus is shown as the area below the demand curve and above the market price line. If the price falls, the surplus of existing buyers increases and new buyers join; if the price rises, the opposite happens.

Source: Britannica: consumer surplus