A stop order becomes a market order when a stop price is reached. 

A buy stop order is entered at a stop price above the current market price, while a sell stop is entered at a stop price below the current market price. Sell stop are order types generally used to limit a loss or to protect a profit on a stock position. 

To place a stop order on web, choose "Advanced Orders" and select "Stop" and on app, select the Stop Buy (or Stop Sell) order from the available drop-down. You can then enter the amount of dollars (for Buy Stops) or shares (For sell Stop) to trade and the stop price you want the market to trade at before your stop market order is executed.  Please note that the stop price needs to be 5 cents below (for sells) and above (for buys) the current market bid or offer respectively. 

All Stop Orders remain queued until it is executed or your delete it before that time.