The order book stacks bids and asks by price level, showing depth of demand and supply. It's what powers the visible bid/ask spread.
Retail traders rarely look at the full book, but market makers use it constantly to price and route orders.
Practice this in Aurora
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Open Decision Lab →Frequently asked questions
Can I see the order book?
Some brokers show 'Level 2' data for a fee. For long-term investing it's almost never needed.