Trade policy governs how freely goods and services cross borders. Tariffs raise the price of imports to shield domestic industry; open trade lowers consumer prices and expands export markets. Both deliver real benefits and real costs, distributed to different people.
Economists broadly agree that open trade raises total output, because countries specialize where they are relatively most efficient. That aggregate gain coexists with concentrated losses in industries and communities exposed to import competition.
The distribution is the political core of the issue. Gains from cheaper goods are spread thinly across all consumers; losses are concentrated in specific towns and sectors, where they are severe and often long-lasting.
Trade policy also serves non-economic goals: securing supply chains for critical goods, applying pressure through sanctions, and reducing dependence on strategic rivals. These considerations can point in a different direction than pure efficiency.