The concept of deadweight loss
The idea behind deadweight loss is that when disruptions to the free market occur, they result in an inefficient allocation of resources compared to the regular equilibrium. Government action is one common source of those disruptions, either through direct control of markets, such as price ceilings, or through taxes that effectively raise the price of goods and services.
In supply and demand terms, when the government imposes a tax, the higher price reduces consumer demand. That, in turn, leads to a new equilibrium that involves less production volume. Despite the higher price, the sum of the money collected in the tax and what the producer receives for the goods is less than the value of the goods produced and sold in the absence of the tax.