supply curve
A supply curve is the seller's side of the picture, the twin of the demand curve. Ask producers, "At each possible price, how much would you offer for sale?" and chart the answers — price up the side, quantity along the bottom. Because sellers offer more when the reward is higher, the line climbs from lower-left to upper-right. That upward slope is the law of supply drawn out: higher price, larger quantity supplied.
The curve carries a quietly important second meaning. Read upward from a quantity to the price needed to coax out that unit, and you are reading the cost of producing one more — the marginal cost. The supply curve is, in effect, a marginal cost curve: it shows the lowest price at which a seller will part with each successive unit, which is roughly what that unit cost to make. As with demand, a supply curve assumes everything else is fixed — the same technology, the same input prices, the same set of sellers — and it lives in one market over one stretch of time.
Once more the key skill is telling a movement along the curve from a shift of the whole curve. If the good's own price changes, you slide up or down the existing line: that's a change in quantity supplied. If costs fall (cheaper materials, better technology), if more firms enter, or if a tax or subsidy lands, the entire curve shifts — right means more offered at every price, left means less. That's a change in supply. Where the supply curve crosses the demand curve is the market equilibrium, the single price at which the amount offered exactly matches the amount wanted.
Cheap solar panels: a new manufacturing technique slashes production costs, so at every price makers will now offer more panels. The whole supply curve shifts right, the equilibrium price falls, and far more panels are sold.
A cost-cutting technology shifts the whole supply curve right — not a slide along it.
The neat upward-sloping supply curve assumes a competitive market of many small price-taking sellers. For a monopolist who sets the price, there is no supply curve in the usual sense — that's a subtlety beyond this field but worth knowing the textbook curve has limits.