The trade barrier described in the text is tariff (D).
A government is establishing tariffs when it is asking other countries to pay a tax every time their companies want this government to buy some of their products.
It is a barrier to trade in the sense that it discourages many foreign companies from selling to Saudi Arabia (the example in question) because they have to pay high tariffs, while other countries might buy without imposing such constraints.
Here, we are are told for instance that the Saudi Arabian government demands that companies which want to sell it food pay 40% of the food's price in tariffs. The goal is to help smaller Saudi Arabian food businesses to sell their products to people in the country, instead of flooding the country with foreign goods.
