The image shows the supply and demand for a product. The original equilibrium price is D, and the original quantity is E. When an import tariff is set, it raises the domestic price of the good to A. This reduces the quantity demanded to G, and it increases the quantity supplied to H.
The additional producer surplus gained is the shaded area F + G + H. This area represents the additional money that domestic producers are able to keep from selling their goods. The tariff raises the price of the good, so producers are able to sell their goods for more money. This additional money is represented by the shaded area.
It is important to note that the additional producer surplus is not a measure of the overall welfare gain from the tariff. The tariff also reduces consumer surplus, and the overall welfare gain or loss depends on the relative sizes of the producer surplus gain and the consumer surplus loss.