Consumer Surplus. Consumer surplus is the difference between what consumers are willing (and able) to pay and what they actually pay. Consumer surplus is the area between the demand curve and the market price.
The demand curve shows the maximum price consumers are willing and able to pay to buy different quantities of a good. Consumers are willing to pay a high price for the first unit they buy but a lower price for each extra unit they buy. Although, consumers only pay the market price P* for each unit.
At the market price P* consumers buy Q* units of output so they spend a total amount equal to the blue area. However, consumers are willing to pay the yellow and blue areas. Consumer surplus is the difference between the two, the yellow area.