A tariff is essentially a tax levied on a product coming into a country. In the US, tariffs are paid at the time goods enter the country. The purpose of the tariffs is to push the cost of the goods coming into the country higher, in turn, encouraging US firms to buy from other US firms rather than buying from a foreign country.
Tariffs can have unintended consequences. For example, a tariff could cause short or limited supplies. If there is a tariff (tax) on goods coming into a country, local companies may be not able to meet increased demand. Another consequence is that the cost from local producers may be higher than the same product sourced from outside the country (before tariff). This could increase the cost of the final product.