When a government limits its trade with other countries, especially by taxing imported goods, it's called protectionism.
A country's protectionism restrains competition from foreign companies. In other words, it may cost more for a US company to make a t-shirt than a Chinese company, but if the US's protectionism means a high tax on the Chinese t-shirts (making them more expensive to US buyers), they won't threaten the American company's business in the same way. Quotas and regulations are other tools of protectionism. Protectionism comes from the idea that the government is protecting its industries.