"Market order" is when you choose to "buy at market price" or "sell at market price".
Market orders will match your purchase or sell transaction according to current market price. Since the market price for digital assets tend to fluctuate, the price will be promptly matched and ordered at the time of purchase or sell; the final price fluctuates according to market orders.
If matched successfully, then the transaction is successful. If it is partially matched successfully, then you will buy/sell the particular portion. The remaining amount will be returned to your wallet.