What is the passage mainly about?

[11] The market is a concept. [12] If you are growing tomatoes in your backyard for sale you are producing for the market. You might sell some to your neighbor or some to the manager of the local supermarket. But in either case, you are producing for the market. Your efforts are being directed by the market. If people stop buying tomatoes, you will stop producing them. If you take care of a sick person to earn money, you are producing for the market. If your father is a steelworker or a truck driver or a doctor or a grocer, [12] he is producing goods or service for the market. When you spend your income, you are buying things from the market. You may spend money in stores, supermarkets, gas stations, and restaurants. Still you are buying from the market. When the local grocer hires you to drive the delivery truck, he is buying your labor in the labor market. The market may seem to be something abstract. But for each person or business who is making and selling something, it’s very real. [13] If nobody buys your tomatoes, it won’t be long before you get the message. The market is telling you something. It’s telling you that you are using energies and resources in doing something the market doesn’t want you to do.