Supply and demand is the basic economic model behind how a competitive market actually sets a price, treating buyers and sellers as two opposing forces pulling in different directions. Demand describes how much buyers want to purchase at each possible price, more at lower prices, less at higher ones, while supply describes how much sellers want to offer at each possible price, more at higher prices, less at lower ones. A market settles at the single price and quantity where these two opposing curves cross, the point where the amount buyers want to buy exactly equals the amount sellers want to sell.
Prices above or below that crossing point create pressure that pushes the market back toward it
If a market's price sits above where supply and demand actually cross, sellers want to offer more than buyers want to buy at that price, creating a surplus that puts downward pressure on price as sellers compete to clear their unsold stock. If price instead sits below the crossing point, buyers want more than sellers are willing to supply, creating a shortage that puts upward pressure on price as buyers compete for the limited available quantity. In both cases, that pressure pushes the market's actual price back toward the point where supply and demand exactly balance, which is exactly why that crossing point is called the market's equilibrium.
A shift in either curve moves the whole equilibrium, not just one side of it
Because equilibrium price and quantity are set by where the two curves cross, anything that shifts either curve, a change in buyers' income or tastes shifting demand, a change in production costs or the number of sellers shifting supply, moves that crossing point to a genuinely new equilibrium, changing both price and quantity together rather than one in isolation. This is exactly why economists reach for supply-and-demand analysis to explain real price movements: a price rise can be read back to figure out whether demand grew, supply shrank, or both moved at once, simply by tracking which curve, or curves, actually shifted.
What we're still unsure about
That competitive markets tend toward the price and quantity where supply and demand cross is well established, extensively tested economic theory, confirmed across countless real markets. What's more genuinely an ongoing empirical and theoretical question is exactly how well this simple model predicts price behaviour in markets that depart from its underlying assumptions, markets with only a handful of sellers, markets with significant information gaps between buyers and sellers, or markets where prices are legally restricted, since real-world frictions can meaningfully slow or block the adjustment toward equilibrium the basic model assumes happens cleanly, and economists continue to develop and debate extensions to the basic framework for exactly these more complicated cases.
This sits inside Supply & Demand, one of eight topics in Microeconomics, one of five domains in Economics, one of seventeen subjects the app can quiz you on.