A stop order is used to trigger at the specified price. Once a stop order triggers at your given price, a market order is executed, immediately matching the highest bids/offers in the order book, in sync with other orders being executed.
Because of the market order’ nature, there's a chance it will execute at a price above or below your stop trigger price. It might happen due to slippage.
The benefit of a simple stop order is that the order is fulfilled no matter what, including significant trend reversals. But there is a possible downside: price control during a fast-moving event can potentially affect the average execution price. During a brief, rapid change in price/volume, such as a fundamental market event that results in increased buying/selling at an unusual rate, a large stop order is triggered to execute a market buy/sell or a large position entering liquidation. In such a case, the stop order may be triggered and begin executing a market order at a significantly different price from the trigger price.
If you prefer tighter control over your average execution price, consider a stop-limit order. It will allow you to set a stop trigger at a specific price, as with a traditional stop; however, you also set a limit price until which the order may be executed. The risk with a stop-limit order is that if the market reverses the trend significantly and does not recover, your limit may restrict the execution of having the order fulfilled in its entirety.
You can learn more about a stop-limit order here.
If you have any questions, please feel free to reach out to Bitfinex Support. We are happy to help!