It's the simplest way of trading. Rather than naming a price and quantity a 'Market Order', also known as "Instant Order", only names the quantity. Your order will be executed at the best market price available in the order book at the moment of making the order.
"Market price" for an order means the lowest current ask price (for buy orders), or the highest current bid price (for sell orders).
When you create a market order, the trade engine will immediately buy/sell at the best price available until either your order is filled or you run out of funds. Since the order is immediately matched it will not appear in the order book or your open orders. Market Orders can not be cancelled.