What is meant by comparative advantage?

Comparative advantage refers to a country's ability to produce a good or service at a lower opportunity cost than another country. Opportunity cost is the 'next best alternative forgone'. So for simplicity let's imagine that in a country's economy only two possible goods are produced, cola drinks and pizza. So, for example if the country wants to produce and additional unit of pizzas, it has to produce 3 units less of cola drinks. The opportunity cost of producing an extra unit of pizza is 3 cola drinks. This because the country is having to forgo producing 3 units of cola. Imagine there are two economies producing both pizza and cola drinks. Country A must give up 3 units of cola drinks to produce an extra unit of pizza. Country B however, only has to produce 1 less unit of cola drinks to be able to produce an additional unit of pizza. Therefore, in this scenario, country B has the comparative advantage in producing pizza. This because country B can produce pizza at a lower opportunity cost.