A market order is an instruction to buy or sell an FX instrument straightaway. The order assures immediate execution but does not guarantee a price. A market order typically fills near the ‘Ask price’ for a long position and near the ‘Bid’ for short.
For instance, assume that the bid/ask for the AUD/USD pair is 0.9560/0.9565. If you place a market order to buy AUD, the order fill would be at or close to 0.9565. On the contrary, if you place a market order to sell AUD, the order fill would be at or close to 0.9560.