The live list of resting buy and sell limit orders at each price level for an asset.
The bid-ask spread. Buy orders sit below, sell orders above, and the gap between the best bid (50.01) and best ask (50.04) is the spread you pay to cross. Bar length shows the size resting at each price.
An order book shows every limit-order waiting to be filled, grouped by price. Bids stack below the current price; asks stack above it. The top of each side is the bid and ask.
Watching the book gives a rough sense of where supply and demand sit, but it is not a prediction. Large orders can be pulled at any moment, and iceberg-orders hide most of their size.
Example: the book shows 2,000 shares bid at $20.00 and 400 shares offered at $20.01. A market buy for 1,000 shares takes the 400 at $20.01 and then walks up to the next level, producing slippage.