Countries trade with each other because specialising in what they produce at a lower opportunity cost, then exchanging goods and services, can let both countries consume more than they could without trade.

Cambridge International AS Level Economics: this answer covers 6.1.1 (absolute and comparative advantage), 6.1.2 (specialisation, free trade and the trading possibility curve) and 6.1.4 (limitations of these theories) in The reasons for international trade.

Why can both countries benefit?

Opportunity cost is what must be given up to make something else. A country has a comparative advantage when its opportunity cost of producing a good is lower than another country’s. That is different from absolute advantage, which means being able to produce more with the same resources.

Suppose Country A can produce either $10$ shirts or $5$ bags, while Country B can produce either $6$ shirts or $4$ bags. A can produce more of both, so it has an absolute advantage in both. But making $1$ bag costs A $2$ shirts and B only $1.5$ shirts. B therefore has a comparative advantage in bags; A has one in shirts.

If A specialises in shirts and B in bags, they can trade. For example, exchanging $1$ bag for $1.75$ shirts benefits both: A gives up fewer shirts than it would take to make the bag itself, while B receives more shirts than it could make by giving up a bag. A trading possibility curve shows how trade can allow consumption beyond a country’s own production possibility curve, which shows what it can produce using its resources.

Free trade means trade without barriers such as tariffs. It can make specialisation and exchange easier, but the gains are not automatic. Transport costs, trade barriers or resources that cannot readily move into another industry may reduce them.

Exam mistake: do not say that only the country with the absolute advantage can benefit. The key comparison is opportunity cost.