An order book displays all buy and sell orders an exchange has for a certain trading pair. The top section of an order book displays the "Sell" orders, also known as "Asks". The bottom section shows the "Buy" orders, known as "Bids".
Tip: You can click an item in the order book to pre-fill the price in the order entry form as a convenient way to enter the price at which you'd like to buy/sell.
In the middle, the last price at which an order was filled is shown. In the example above, this is 3647.48 for BTC/USDT at Binance. The value shown next to it is this price converted to your preferred currency (in this case USD). Since the price of USDT is not always exactly 1 USD, the converted price is usually different.
For each row in the order book, you can see the Size or the amount available at the Price shown, and the resulting total (price x size). This gives you a sense for the depth of the order book.
As visual guide, the green and red bars on the left show the relative size of each price/size entry in the order book. The larger the bar, the greater the combined size of the orders at a certain price point. This is a convenient way to visualize any buy/sell walls.