A Market Order is an order to immediately buy or sell an asset at the current market price. Its advantage is the immediate execution of the transaction; however, it does not guarantee the specific price you will buy/sell the asset.
How it works: For example, you want to buy Bitcoin for $10,000. Instantly. Here and now. Assume that the current BTC market sell rate is $10,000 for 1BTC. Ideally, the exchange transaction will get you 1BTC. But! The sale takes place at the best available price in the current market. There may not be any offers for one bitcoin at this price in the order book from other exchange participants. Your order will automatically buy bitcoin from more expensive orders, with the price progressively growing until BTC has been purchased for the whole amount you specified.
Therefore, you will instantly get Bitcoin, but the final buy price will be different from the market price that was current when you clicked the ‘Buy’ button. In our example, you may get 0.95BTC instead of 1BTC, while the average buy rate is $10,526 for 1BTC.