Explain Like I'm Five

When you enter the United States, customs "inspects" all the stuff you're bringing back. If it's more than $850 worth of stuff, then you have to go to the cashier and pay a tax.

The tax is a percent of what the stuff is worth. The percent rate can depend on what type of goods it is, and what country it's coming from. There are massive tables to look this stuff up.

The stuff you carried out of the country and are now bringing back with you doesn't count toward the $850 limit.

If you're shipping stuff in but not traveling with it, there is no exemption. Tax applies right away. You also have to hire a guy called a broker to help you with the CBP paperwork and to submit payment.

So let's say somebody is importing sugar from the Caribbean, and there's a tariff. They have to pay a percent to the feds every time they ship in some sugar. They raise the price they charge on the sugar to cover that. Then sugar from Louisiana looks more attractive on the store shelf because it's cheaper.

Who pays? Whoever is shipping the goods in pays, but they make it up by charging more for the imported products.

Why do it? Usually, you want to make some domestic industry more attractive by raising the price of the foreign competition.

In the sugar example, sugar is more expensive to farm in Louisiana because people get paid more, and the equipment is more expensive. If there wasn't a tariff, people might stop farming sugar in Louisiana entirely. That might make some people sad. On the other hand, all Americans would be able to pay less for sugar without the tariff.