The correct answer to this open question is the following.
Trade in the early empires represented the opportunity to exchange products that ancient people needed with other towns or city-states, as was the case of Sumeria. City-states such as Kish, Uruk or Ur traded between them and years later they reached as far as the Indus Valley Civilization. Years later, the Silk Road represented the way to trade in those far lands from China to Africa and some parts of Europe.
Modern-day trade is more complex because other countries have their own rules and regulations on trade, imports, and exports. The international c}urreny is the American dollar. To eliminate barriers that impede free trade, countries had come up with the idea of free trade agreements such as the North American Free Trade Agreement signed by México, the United States, and Canada.