A market is an order to buy or sell at the current market price. A market order will be once it is matched with the relevant buyer or seller. However, please note that while the of the order is certain, market do not guarantee the price at which they will be executed. 

For example, you place a market order to buy shares of a company. While your order might be executed immediately, there’s no guarantee you will buy the stock at the last traded price when the order was placed. Instead, it may be bought at a price closer to it. This slippage is because the market is volatile and the last-traded price may have already changed by the time the order is executed.