A tariff is a kind of tax on goods a country imports or exports. If you want to buy a European-made car in the U.S., the price will include tariffs the government adds to the price of imported vehicles.
Usually a government imposes a tariff to encourage its own industries and to discourage buying cheaper imports from other countries. If a government wants to protect its own clothing industry, it may add a tariff to imported clothing, to make sure that the imported clothes aren't cheaper than the locally manufactured items. You can control exports, too, by imposing tariffs. As a verb, you can say "the government tariffs certain imports and exports."