An order book is an electronic list of all the buy and sell orders organized by the price level on an exchange. The real-time register represents the interest of buyers and sellers. A matching engine uses the book to determine which orders can be fully or partially executed.
The buy orders are shown in green in the book, while the sell orders are in red.
Passive orders, like Limit or Conditional Orders, which are executed at a specific price or better, stay open in the order book and show the market depth of the exchange. The passive orders don't move the market price.
Active Market orders are executed almost instantly and directly influence the Last Traded Price. The Last Traded Price, the Index Price, and the Mark Price are displayed in the order book on BTCMEX.
An order book shows the trigger price at which an order must be executed (Price), the number of contracts available at that price (Size), and the total number of all the available contracts at that price or worse (Total).
BTCMEX explained What is the Better/Worse Price on the exchange.
The order book is customizable on BTCMEX. You can group orders by the price interval to see the market depth clearer, and switch between single or dual columns. Liquidation alerts are also available here.