Any item that is readily accepted by people in return for goods and services.
Exchanging one set of goods for another. Can result in complicated transactions: how many shells would be equal to 4 shiny rocks?
When producers and consumers agree to provide one type of item in return for another. If Apple charges $600 for a new iphone, and very few people buy them then the “producers” of the iphone (Apple) know that consumers are not willing to engage in that exchange.