A Stop Order is used to buy/sell at a price above or below the current price. A stop order has two steps to be executed, which are:
Trigger- the security’s price trades at or through the stop price, triggering/activating the order ; and then
Execution- the stop order becomes a market order and is executed at the next available price, completing the trade.

***Note: The entry of Stop Orders is available 24/7. However, Stop Orders will only be executed during the normal NYSE market hours (9:30 AM to 4:00PM Eastern Time Monday through Friday). There is no guarantee that if a stop order is triggered, the investor will pay or receive the stop price.

Execution is usually certain, but price is not. In most cases, your order will be executed at a price that is close to the market price at the time the sell stop order is triggered. However, as an example, if a stock’s trading is halted and the stock price gaps down upon reopen (lower than the price at the time trading was halted), your execution price could be significantly lower than your sell stop price.