The order books let you see how many orders are available in different price ranges.
The Asks order book displays orders from people who wish to sell Bitcoin. The number in the middle is the price and the number on the left is the volume.
The Bids order book displays orders from people who wish to buy Bitcoin. The number in the middle is the price and the number on the right is the volume.
The Last Price field between the order books displays the most recent executed price as well as the spread. The spread is the difference between the best bid and the best ask displayed as a percentage.