The order book is the central record of a market's open orders. Buy orders, called bids, are listed below the current price, and sell orders, called asks, are listed above it. The highest bid and lowest ask define the best available prices, and the gap between them is the spread.
Each price level shows how much volume is waiting to trade there. Reading this depth gives a picture of where buying and selling interest is concentrated and how much size it would take to move the price through those levels.
Order books are dynamic: orders are added, cancelled, and filled continuously. Because resting orders can be withdrawn at any time, the visible book is a snapshot of current intent rather than a guarantee of future trading.