A market order is an order that is sent to be executed at the current market price, regardless of what that price is when your order is executed. The advantage of using a market order is you are pretty much guaranteed to be filled on your order, regardless of how fast the market is moving. The disadvantage is that you do not know where your order will end up being filled, until after you receive confirmation from your broker of the fill price.
It is important for traders to understand that just because they click to buy or sell at a certain rate on their platform this does not mean that they will be executed at that price. The difference between the price a trader clicks to buy or sell a stock, and the price that they are filled at on their trade, is called “slippage”. Factors including the volatility or amount of price movement in the market at the time the order is placed, the time of day the trader is trading, and how active the stock is that they have chosen to trade, affect the quality of execution that a trader will receive on their orders. In general, the more active the overall market is, and the greater the amount of volume traded in a stock on average, the more liquid a stock is, and the better the execution or “fill” a trader can expect.