The document discusses the demand function and why the demand curve slopes downward. It explains that the demand function shows the relationship between the quantity demanded and the price of a product. Price is one of the most important factors influencing consumer demand. The demand curve slopes downward for three reasons: 1) the law of diminishing marginal utility, which states that marginal benefits decrease as consumption increases, so consumers will pay less; 2) the income effect, which suggests that lower prices increase consumers' real income and thus their demand; and 3) the substitution effect, where lower prices make a good relatively cheaper compared to alternatives, causing consumers to switch to the more affordable option.