President Trump announced what could be one of the most dramatic economic policy changes in decades on Wednesday, when he substituted America’s longstanding system of taxing imports with a new tariff system of his own devising.

The president said the tariffs would reverse decades of unfair treatment by the rest of the world and result in factories and jobs moving back to the United States.

“The markets are going to boom” and “the country is going to boom,” Mr. Trump said on Thursday, as global financial markets suffered their biggest rout in years. He added that other countries “have taken advantage of us for many, many years.”

Economists’ estimates have been far more grim, with most predicting that the president’s sweeping tariffs and likely retaliation will slow U.S. economic growth, push up costs for consumers and make life difficult for businesses that depend on international supply chains.

The president’s measure is both consequential and complicated. Here’s what you need to know.

Mr. Trump announced two big tariff plans that apply to most of the world. One component is a “base line” tariff of 10 percent that will apply broadly to nearly all U.S. imports, except for products coming from Canada and Mexico.

The second measure is what the president is calling a “reciprocal” tariff. That levy will apply to 57 countries that Mr. Trump says have high tariffs and other unfair economic practices that have hurt American exporters. He said this is a reciprocal tariff because it will match the way other countries treat the United States.

Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.