@SarahGen-- It's easier to understand comparative advantage when it's compared to absolute advantage.
In absolute advantage, a country produces and trades a good when it's more efficient at it than any other country. In comparative advantage, countries produce a good that it is more efficient in, when compared to other goods.
So, for example, lets say that we have country A and B. Country A produces berries and oranges more efficiently than country B but it's most efficient in berry production. Country B is less efficient at both when compared to country A, but it is slightly better at producing oranges than berries. According to comparative advantage, country A should produce berries, country B should produce oranges and they should export them to one another.
If we were using absolute advantage, we would have country A produce both berries and oranges. Country B would not be able to export or import either and Country B would only be able to export those goods. So comparative advantage encourages trade and it creates a situation where everyone has something to gain.