When a Stop triggers at your specified price, a Market Order is executed and immediately begins to match the prevailing bids/offers available in the order book, in-line with other orders being executed.
The nature of a Market Order can increase the risk of the order performing at an average execution price above or below your Stop trigger price due to slippage.
The benefit of a simple Stop is that the order is fulfilled no matter what, including significant trend reversals, but the downside is that price control during a fast-moving event can potentially affect the average execution price. During a momentary, rapid price/volume change, such as a fundamental market event causing increased buying/selling at an atypical rate, a large stop order being triggered to perform a market buy/sell, or a large position entering liquidation, the Stop could be triggered and begin executing a Market Order at a price that is different to the trigger price .
If you would prefer tighter control over your average execution price, consider a Stop-Limit order. This will allow you to set a Stop trigger at a specific price, as with a traditional Stop; however, you also set a LIMIT price at which the order may be executed until. The risk with a Stop-Limit order is that if the market reverses trend significantly and does not recover, your limit may restrict the execution of having the order fulfilled in its entirety.