Imagine this:a robboer goes into a bank and deemands some money. They give him a shell, four goats, six tomatoes and eleven chickens. Before the invention of coins and peper currency, people used such things for buying and selling. This exchange of goods and services for other goods and services is called barter.
Around 9000 BC people paid for goods with animals like cows, sheep and camels. Some societies used vegetables and plants, too.Then in 1200 BC people in China started using shells as money - mainly the cowrie shell. Soon other countries began to do the same.