Market orders let you buy or sell a stock immediately at the best available price at the time of submission. If the market is open, the order will be submitted right away and will fill at the best available price contingent on the liquidity at each price level.
Due to this, please keep in mind that the last-traded price is not necessarily the price at which your market order will be executed, and that there could be fluctuation between the executed price and the last seen price.
Note: Our executing brokers are obligated by law to give you the best available price based on the National Best Bid and Offer (NBBO). If the market is closed, the order will be filled once the market reopens.