What is a economic theory in which nations try export more goods then they import?

The economic theory you're referring to is known as mercantilism. This theory suggests that a nation's strength is directly related to its wealth, which can be increased by maximizing exports and minimizing imports. Mercantilist policies often involve protectionist measures such as tariffs and subsidies to promote domestic industries and maintain a favorable balance of trade. While influential in the past, mercantilism has largely been replaced by more modern economic theories that emphasize free trade and comparative advantage.