The amount of a good or service firms plan to supply to the market at a given price.
Below is an illustration of a typical upward sloping supply curve. This shows that when the market price for a particular good begins to rise this causes the quantity supplied for that good to rise. This is because firms have a profit incentive to sell more goods at higher prices. But also if they are to produce more goods, production costs rise in line with that and therefore they have to charge a higher price to maintain margins.