A stop price is the price in stop order that triggers creation of market order.
In the case of a Sell on Stop order, when the market reaches or falls below the Stop Price a market sell order will be triggered. For Buy on Stop orders, the stop price triggers the creation of a market buy order when the market price of the stock rises to or above the stop price.
In addition if a Stop Limit is also indicated on stop order, the resultant order will be a corresponding limit order as opposed to a market order.
References
Stop price Wikipedia(Text) CC BY-SA