A stop-loss order is used to limit your loss on a position in a security. The stop order can be used on both long and short positions.
If the order is used in connection with a long position, a market order to sell is triggered at a certain price - the order will get filled at the available price in the market.
Note: Execution is not guaranteed.
To place a stop-loss order on a particular position you need to right-click on the position you intend to place the stop-loss order in relation to. In the menu you need to choose Take Profit / Stop Loss, which will open a new window. In the Stop Loss field you need to indicate the price at which your stop order will be placed (below the current price for long positions and above the current price for short positions).