The land of Mesopotamia did not have a lot of natural resources, or at least they did not have the ones in demand during that time period. So, to get the items they needed the Mesopotamians had to trade.
In the southern part of Mesopotamia, docks were built along the sides of the rivers so that ships could easily dock and unload their trade goods. The merchants traded food, clothing, jewelry, wine and other goods between the cities.
Sometimes a caravan would arrive from the north or east. The arrival of a trade caravan or trading ship was a time of celebration.
To buy or trade these goods, the ancient Mesopotamians used a system of barter. But they also used money. They didn't use paper money or coins. They used barley for local trade. Because barley was heavy, they used lead, copper, bronze, tin, silver and gold to "buy" things away from their local area. You had to borrow barley from a barley banker. The banker charged very high interest.