This was used to protect American industry. What is it?
This question is a little vague, but it seems certain that you are asking about tariffs, since they have historically been the primary means used by the federal government to protect domestic industry. Tariffs, sometimes called "protective" tariffs, are taxes, or duties, placed on imported goods. This is intended both to raise revenue (before income taxes they were the main source of income for the federal government and to make domestic products more competitive on home markets. They do this because the tax on imports, paid by importers, is eventually passed on to consumers. Therefore the imported good will be more expensive than one manufactured domestically, since there is no tariff on domestic goods. High tariffs have been controversial in American history, though, because they tend to raise prices overall. They also can hurt American exports, since other countries often respond to the passage of tariffs in other countries with tariffs of their own. Tariffs were a major political issue in the nineteenth century, and in 1833 the state of South Carolina (with a primarily export economy) claimed the right to "nullify" a particularly high tariff. The income tax, made constitutional by the Sixteenth Amendment under Woodrow Wilson, was created as a means to lower tariffs by providing an alternate source of federal revenue.