Tariffs are a tax on the American consumer. Say a 25% tariff is levied on Chinese widgets. That 25% tax is paid by the American importer when a shipment of Chinese widgets arrives in the country. The importer then passes that additional expense on to distributers and/or consumers in the form of higher prices for the widgets. So, the American consumer ends up paying more for the widgets. Then what happens is fewer consumers buy widgets due to elevated price. They may buy another widget from another country that isn't under tariff, or from a domestic manufacturer if one makes comparable widgets. That's the basis for the claim that tariffs will boost domestic profits. But that's assuming America makes a comparable widget for a lower price than the tariffed widgets, or widgets from non-tariffed nations.

Where it gets more consequential is when a tariff is imposed on a raw material that America can't produce, or at least not in sufficient quantities to meet the demand. Like steel and aluminum. America buys a lot of steel and aluminum from Canada because we can't produce enough to meet the demand. We previously had free trade with Canada, so steel and aluminum could enter America without any additional expenses, meaning it was available to consumers at the lowest possible price. But with a 25% tariff, everyone in America who uses steel or aluminum in their products is going to have to pay more because there isn't a cheaper domestic option available. The result is that everything that uses steel and aluminum is now significantly more expensive. And that suppresses demand, and consumer buying which has an adverse impact on the economy.

There are more complexities to it, but this is a good view of the fundamentals.