Money. Very early in history, all trade was in exchange of goods.
Cattle was exchanged for grain, manufactured goods were traded to agricultural products. However, it was often difficult to determine what the value was of something, and often someone who wanted something had nothing that the owner would accept in return.
For centuries, the value of things was expressed in cattle. Until someone realized that it would be more convenient to express that value in a precious metal, gold or silver.
For example, gold and silver were used in exchange and to avoid weighing the quantities of metal each time, standard coins of that metal were introduced.
Later on debts were made exchangeable for coins at exchange locations, and finally banknotes were put into circulation which were accepted by created banks as money.
In the early days banks needed to have large sums of gold and silver in a vault. At any time anyone could exchange their banknotes for its value in gold. Nowadays the whole bank system is based on trust: banks don’t have the value of every banknote in gold anymore, but we trust our Governments that the value of our money is guaranteed.
Now we slowly make money a digital currency: money is more and more exchanged online. But the principle remains the same: our bank still guarantees the value of whatever number we exchange from one account to the other.