producer surplus
Producer surplus is the seller's version of that "I got a deal" feeling. It's the gap between the lowest price a seller would have been willing to accept for something and the price they actually got. If a craftsman would happily sell a chair for as little as 40 dollars (that covers his costs and effort) but the market price is 70, he pockets 30 dollars of producer surplus — money over and above the least he'd have taken.
Sum it across all the sellers in a market and you get total producer surplus, a money measure of how much benefit sellers collectively get from being able to trade at the market price. On the diagram it's the mirror image of consumer surplus: the area above the supply curve and below the price line. The reason is the same in reverse — the supply curve traces each seller's minimum acceptable price (roughly their cost of making each unit), from the lowest-cost producer (down low) up to the marginal seller (right at the price). Everyone whose cost sits below the price earns the difference; the lowest-cost sellers earn the most, and the marginal seller, who'd just barely accept the price, earns almost nothing.
Producer surplus is the second half of total surplus, and pairing it with consumer surplus lets economists weigh the full welfare effects of a market or a policy. Raise a price with a price floor and producers may gain surplus while consumers lose it; impose a tax and both sides typically lose some. Two honest notes: producer surplus is closely related to, but not exactly, profit — over the long run, after accounting for all fixed costs, it lines up more nearly with profit, while in the short run it can exceed it. And like consumer surplus, it's measured in money, so it tells you about efficiency and the size of the pie, not about how fairly that pie is shared out.
Three farmers can grow wheat at costs of 2, 3, and 4 dollars a bushel. At a market price of 5, the first earns 3 dollars of surplus, the second 2, the third 1 — together 6 dollars of producer surplus, on top of covering their costs.
The gain each seller gets above their cost, summed — the area above supply, below the price.
Producer surplus is not the same as profit, though they're cousins. In the short run it counts only the variable costs the supply curve reflects, so it can sit above profit; over the long run, once fixed costs are covered, the two converge more closely.