Instead of trading the current market price, you can place a limit order.

If you use a buy limit order, you are saying that you don't want to buy the stock until its price has fallen to a specific level. Let's say that you are interested in buying Harley Davidson stock, which is currently trading at 44.00. But, you think the price will go somewhat lower and therefore you don't want to buy at the current level, but at 42.00. Instead of having to watch the screen and wait for the market price to descend to 42.00 (if it ever does), you can place a limit order to buy at 42.00 that will automatically execute when the market reaches that price.

Note that the trade is only triggered when the market reaches that price, it does not guarantee that your trade will be executed at precisely 42.00. Especially in fast moving markets, the actual price at the time of execution may be slightly or significantly lower than specified in your limit order, depending on the speed of the market's movements.