Although the use of the stop loss feature is different between platforms, it is used in a similar way on many platforms. In order for us to be able to help you, I think it would be better to specify which platform you would like to use this feature on. To give very brief information about the stop loss feature in general;

A stop loss order is an order sent to the broker to sell a security when its price reaches a predetermined level. It is often associated with long positions. An investor in a long position has invested in a security and holds it. If the price drops to a certain level, stop loss will be activated and the position will be closed by selling the relevant security.