It means that the status of marijuana is based on consumer preferences. Consider, for example, a society in which large numbers of people love marijuana and are addicted to it. There the demand for weed would be high but the supply would be limited since the production of the weed is illegal and must be sold undercover and on the black market, away from prying eyes. Likewise, in a society with few marijuana users, where there is a low number of drug users and pot buyers, there may be a surplus in the market since not many people would buy pot. If the federal government were to allow the production of marujuana, there would be a drastic change in prices and quantity sold on the market. As when pot was illegal, it had to be sold on the black market, which meant its price was high and the quantity supplied limited. But in the case of a market where pot production is legal, the price and quantity will be determined based on the forces of supply and demand, meaning that prices will realistically be lower and supply may be reduced.