Stop loss is an order for closing a previously opened position at a price less profitable for the client than the price at the time of placing the stop loss. Stop loss is a limit point that you set to your order. Once this limit point is reached, your order will be closed. Please note that you need to leave certain distances from the current market price when you set up stop/limit orders.
Using stop loss is useful if you want to minimize your losses when the market goes against you. Stop loss points are always set below the current ASK price on BUY, or above the current BID price on SELL.