In the different subsistence modes, surplus plays a critical factor in an economic system. I will clarify my statement.
Foraging (Hunting & Gathering): Foods have a short shelf-life and will spoil rapidly if not preserved or eaten. So, people either kill or harvest exactly what they will eat when they need it, or they sell or give away any extra they don't consume. The extra is called surplus and is the basis of economics. I gather enough for myself and family and sell the surplus to you. In turn, you gather a different product and sell any surplus to me.
Cultivation (Horticulture & Subsistence Agriculture): In the beginning, farmers planted crops to provide for themselves and their animals. They initially grew enough to eat fresh during the summer and preserved the rest to carry them through the winter months. Then, they discovered that if they planted extra of their crop (surplus), they could sell it and make a profit. Farms grew larger and larger to produce more and more surplus to make more and more money!
Pastoralism (Nomadism & Ranching): Whether people stay in one place to raise animals (ranching) or follow the animals around from pasture to pasture (nomadism), the animals are bred for one purpose: to provide people with food, clothing, and shelter materials. Some animals were huge and had more meat than could be consumed by one person! So, the extra was surplus and was sold to many people. The raising and selling of animals for profit has became a very lucrative business.
Distribution & Exchange: As more and more surplus products became available, there needed to be better ways of getting the products to the people who needed them. So, companies were formed to distribute (or spread) the products around the country (or world). Today's distributors have the means to buy, preserve, store, ship, and sell the surplus products that others make. Sometimes distributors will simply exchange one surplus product for another surplus product of equal value.
Surplus has the unique way of regulating the economy in that the more of something there is, the lower the price will be for that item. It keeps prices low and the economy healthy.