Market order (instant order) involves buying or selling cryptocurrency immediately at the best available price. On CEX.IO for buying with a market order, you select the amount of FIAT to spend (i.e. "Buy me enough bitcoins for $100 right this moment"). Selling with a market order is the same as Limit Order, except the best available price is used automatically
The best price is calculated from the top of the order book down. For example you would like to sell 5 BTC with a market order. There are currently buy orders for 3 BTC at $600, and 4 BTC at $550, which represent the best prices available. Your market order will match the first one completely, giving you $1800 for 3 BTC, and the second one partially with $1100 for 2 BTC. In total - $2900 with an average price of $580 per BTC.
Market order is executed instantly in most cases (only a complete absence of orders in the order book will leave it hanging) and cannot be reversed.