There are two types of orders available on this exchange: market orders and limit orders:
A limit order will either match existing orders, or will stay on the orderbook.
A market order will either match existing orders, or will not be executed.
For a market order to execute, there must be an equal amount of limit orders on the order book, at the opposite site (i.e. for a market bid, there must be sufficient limit asks, and for a market ask, there must be sufficient limit bids). If there are no sufficient limit orders on the order book, the market order cannot be executed.
Hope this helps. Please let me know if you have any other questions.