The gov't can impose limits on prices, either a ceiling (cannot go above) or a floor (cannot go below, such as minimum wage). When the free-market is prevented from buying/selling all they want, inefficiency or a deadweight loss is created (loss of consumer/producer surplus). Let's look at a price ceiling. Say a society fears that avocado prices are rising too much and it would benefit the people if a price cap was imposed. We all want lower avocado prices! Say a $2 price ceiling is imposed. This is below the "normal" $3 market rate. This would appear to benefit consumers. However, if suppliers can't get a higher price, they will not offer as many. They just can't get excited growing them for such a low price, and feel they won't make a profit. The graph shows what Qty both demander and supplier desire. At $2, consumers want 4 avocados; suppliers will offer only 2 avocados. So we settle at 2 units (both parties can do that - demanders will buy 2 even though they want 4 units). So market (imposed) price is $2 and Qty is 2 avocados. Don't get confused by the graph - this is only an adjustment