Market equilibrium is the price at which the amount buyers want to buy exactly equals the amount sellers want to sell — no shortage, no leftover stock. Think of a concert promoter setting ticket prices: price them too low and fans are turned away at the door; price them too high and seats sit empty on the night. The price that just fills every seat, with no one left out and nothing wasted, is the equilibrium price.

Formally, equilibrium occurs at the price $P^*$ where quantity demanded equals quantity supplied. If the market price rises above $P^*$, sellers offer more than buyers want — a surplus (excess supply) — and competition among sellers pushes the price back down. If the price falls below $P^*$, buyers want more than sellers offer — a shortage (excess demand) — and competition among buyers bids the price back up. Because both forces point toward $P^*$, it is a resting point: once there, the market has no tendency to move. With linear curves, set the demand equation $a - bP$ equal to the supply equation $c + dP$ and solve to find $P^* = (a - c)\,/\,(b + d)$; substitute back to get the equilibrium quantity $Q^*$. To see this move, shift a curve in the interactive supply and demand model and watch both $P^*$ and $Q^*$ adjust in real time.