The essentials
An order book lists resting bids and asks by price. The spread is the distance between the best bid and best ask, and depth shows available size at each level.
What to consider
Visible liquidity can change or disappear before your order arrives. A large displayed bid is not a guarantee of support; cancelled orders and hidden liquidity make snapshots incomplete.
Put it into practice
Compare depth across nearby levels and estimate slippage for your actual size. Use a limit price when execution boundaries matter more than immediate completion.