Order Book
The order book is the live list of buy and sell orders waiting at each price on an exchange.
Buy orders (bids) sit below the current price and sell orders (asks) above it. The best bid and best ask are the prices you can trade at right now; the sizes stacked at other prices show how much it would take to move the market.
A thin order book moves a lot on a large order, so a coin's liquidity shows up there. Big visible orders are not always real, since they can be cancelled in a moment.