A Stop order is an order to execute a trade when the price level (the market level) reaches a level equal to or lower than the current best price.
If you place a breakout buy and sell order at the close to execute the order at the specified favorable price, and there is a jump at the open, the order will be executed at the same or worse price you specified. For example, if you place a breakout buy order to buy at $10 and the market opens at $12, your position will be open at $12.
