The Orderbook is where all buy and sell orders are displayed. All orders are submitted as Limit orders. A Bid is a buy order, the Bid price is the highest amount you are willing to pay for a particular contract and a particular quantity. An Ask is a sell order, the Ask price is the lowest amount you are willing to receive for a particular contract and a particular quantity.
The best Bid and Ask are 10202 / 10203 with 0.348 / 0.412 contracts on each side respectively. If you wanted to immediately sell 0.348 contracts, you could do so at the Bid price of 10202. If you wanted to immediately buy 0.412 contracts, you could do so at the Ask price of 10203.
You can also click on any level of the Orderbook and the Order Controls widget will auto-populate with the cumulative size and the Bid price. This means that if you click on the 'total' of any particular price level in the Orderbook, it will automatically fill the details into the place order tab, both for 'Quantity' and 'Limit Price'.
Any live unfilled orders are marked with an arrow in orange.