An orderbook is a list of buy and sell orders organized by price levels. An orderbook consists of a list of trader/users open orders. The orderbook is split into bids and asks at various prices (price levels). The orderbook is used to identify market demand and allows traders to see what can be sold and bought at the market. Placing orders in front of other people's orders (higher when buying or lower when selling) will help guarantee that your order/s are filled.
10 levels are visible by default on the orderbook. For those that have larger monitors and powerful computer hardware they can increase the levels in the Settings -> Interface.