The order book is the heart of any exchange, and reading it well is the fastest way to get better at trading. Here is the short version.
Bids and asks
Bids are buy orders, sorted highest price first. Asks are sell orders, sorted lowest price first. The gap between the best bid and the best ask is the spread. On a fresh school coin you will see a bid at $0.9999 and an ask at $1.0001 — one tick each side of the $1.0000 start.
Depth tells you conviction
A tall stack of bids just below the price means buyers are waiting; a thin book means a single market order can move the price a long way. Watch where the size sits before you send an order.
Price-time priority
Orders fill best-price-first, and within a price level, first-come-first-served. So a limit order resting at a good price earns its place in the queue. Patience is an edge.