Supply is how much sellers are willing to put up for sale at each possible price. The core pattern — the law of supply — is that a higher price draws out more output. Think about selling handmade posters at your campus market: if buyers pay $5 each, you might print a handful on the weekend. At$20 each, you clear space in your room, buy more ink, and print boxes of them. Higher reward, more effort — that is the law of supply in one sentence.

More precisely, supply is the entire relationship between price and quantity supplied, holding everything else constant — economists call that assumption ceteris paribus (Latin: 'all else equal'). On a standard market diagram, supply appears as an upward-sloping curve with price on the vertical axis and quantity on the horizontal. It is important to separate two very different things: a movement along the curve happens when the good's own price changes, sliding you up or down the existing curve. A shift of the whole curve happens when something else changes — input costs, technology, the number of sellers, or taxes. If a new printing technology halves your ink cost, you supply more posters at every price, and the whole curve moves to the right.