An order book is every resting buy and sell order for a pair, stacked by price. Buy orders below, sell orders above, and the gap between the best of each is where the market currently sits.
Reading it changes how prices feel. What looks like a single number on a chart is really a queue of other people's intentions, and your order interacts with that queue rather than with an abstract price. Everything else in trading mechanics - spread, depth, slippage - is a description of the shape of this list.
Binance Academy covers this in more depth: https://www.binance.com/en/academy/glossary/order-book
Trading crypto from Dubai since 2019.
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