What is meant by the double coincidence of wants?

Double coincidence of wants is a situation in which the buyer and seller should agree to sell each other’s commodity in exchange even if they are not interested. In this system, goods and services are exchanged in return of goods and services needed by the other person without any exchange of money. But the main deal is that both parties must have the goods and services which the other party is ready to buy, or it is useful for him. For example- A shoemaker wants to buy some rice. In this situation the shoemaker must find a person who is interested in buying his shoes and in exchange he will provide him rice so that both will be able to exchange the goods and services which are related to their needs.