This is the most common type of order. You give your stockbroker an order to buy or sell a specific number of shares for a specific stock. The broker will try to fill it at the best price currently available which is called Market Price. This means the lowest price if you are buying, and the highest price if you are selling. This is the simplest type of order. You are almost guaranteed your order will be executed. The disadvantage could be the price you buy or sell at.
For Example: In a fast moving market you can place an order, for High Five Corporation (HFC) stock, currently selling at $30. By the time the order is executed it could be $40, which might be more than you wanted to spend.