What's the price of bitcoin... right now? That's a loaded question. There is no one price. It depends if you're a buyer, a seller, or just want to know what prices have been trading recently. In any of those three cases, the price is likely different. Let's say BTC has been trading near $9500. If you want to pay $9000, that's great, but you're not going to trade. You submit a "limit order" at 9000.00 and your order gets sent to the "order book".
Below is a picture of an order book. The orders to buy ("bids") are on the left side at lower prices, while the orders to sell ("offers") are on the right side at higher prices. Your 9000 bid is all the way off the screen it's so low. But say I want to bid 9540, while you want to sell at 9565. Those orders show up at the "top" of the order book bc they are the highest price someone wants to bid and the lowest price someone wants to offer. Orders in the order book are "passive" and won't trade unless someone else enters an "aggressive" order.
If an incoming order to buy is at a high enough price (or order to sell is at a low enough price) then it might trade. The buyer would be the "taker" and the resting offer to sell would be the "maker". In the order book shown above, we see there are orders to sell over 14 BTC (column labelled "Total(BTC)") at 9565. If I enter a limit order to pay 9565 for 1 BTC, then this will result in a trade. I could also enter a "market order" to buy (no specified price) and I will trade against the lowers offers available in the order book (again, 9565 in this case).
Those were the basics and apply to one exchange. A combined order book is when you take several order books from different exchanges and display them in one combined (or consolidated) order book. In the example below, you can see the best bids and offers from many different exchanges. In this case, there are collectively more orders, more size, and better prices than you could get by going to one exchange (and one order book). There are not many places you can find a combined order book.
These images came from CoveTrader, a crypto trading analytics tool aimed at active traders of Bitcoin, Ethereum, and other cryptocurrencies. CoveTrade consolidates information from multiple different exchanges into one highly customizable real-time front-end. Visit trader.covemarkets.com today and take a look!