The best prices meet in the middle
The highest bid is the best displayed buying price. The lowest ask is the best displayed selling price. Their difference is the bid-ask spread.
A narrow spread can reduce immediate execution cost, but it does not guarantee deep liquidity.
Rows are available price levels
Each row shows a price and the quantity available there. Cumulative depth adds the size from the best price through that row.
A market order larger than the first row continues into additional levels.
A book is a live snapshot
Orders can be added or cancelled before your order arrives. Displayed depth is useful context, not a promise that every quoted unit will remain available.
Displayed size is conditional liquidity
The best bid and ask show the nearest resting interest at one instant. Depth shows additional quantities at worse prices. Orders can be added, cancelled, or executed faster than a screen refresh, so displayed quantity is evidence of current interest rather than a commitment to remain available.1
Estimate an average fill by walking the intended quantity through successive price levels. Then repeat with less depth. That stress case is more useful than comparing the whole order with only the best price.12
Read changes, not isolated rows
A single large order can look like support or resistance, but it may be cancelled, moved, or replenished. More useful observations include whether depth persists as price approaches, how quickly trades consume it, whether the spread widens, and whether new orders replace executed quantity.1
The book shows limit interest on one venue. It does not show hidden liquidity, activity on other venues, future market orders, or the provider’s mark-price calculation. Use it for execution planning rather than certainty about direction.1