The order book (or orderbook) shows what kind of offers are currently on the market. It gives insights in regards to offer types, fees, offer sizes, and fidelity bonds put up by market makers.
Jam will set up a service that allows you to watch the order book on your node. This removes the trust in a third party. You can view the order book directly in Jam via the "Show orderbook" button on the Earn screen.
There are also public order books that you can inspect, in case you do not have access to your own node: