What is a good economic theory that explains the relationship between supply and demand in a market economy?

One good economic theory that explains the relationship between supply and demand in a market economy is the law of supply and demand. This theory states that the price of a good or service will adjust to bring supply and demand into balance. When demand for a product increases, prices tend to rise, encouraging suppliers to produce more. Conversely, when demand decreases, prices tend to fall, leading to a decrease in production. This dynamic interaction helps determine the equilibrium price and quantity in a market economy.