High tariffs on imported goods negatively impacted the Southern economy prior to the Civil War. A tariff is a tax on an imported good. Ultimately, this increases the price of goods from other countries.
The reason this hurt the Southern economy is because they relied on trade with other countries. Since the US implemented tariffs, other countries did not want to trade with the US. This is because the tariff made their goods more expensive for American citizens, resulting in less of their products being bought. This was one of the most significant issues that split the US between Northern and Southern states.