It is an adjustable order to close the trade when the market moves a specified amount against your position and helps you minimize your losses in case the market moves in the opposite direction to what you expected.
You can set your Stop Loss according to a specific level in the market (Rate) or as a monetary amount, which is also shown as a percentage of your initial investment in the trade window.
In normal market conditions, when the market reaches your requested rate and you have lost the predetermined amount, the Stop Loss order will trigger and automatically close your position.
However, the set Stop Losses are not guaranteed. During volatile periods when spikes, market gaps and slippage may occur due to market conditions, your Stop Loss may not trigger at your requested rate. If your set Stop Loss is not traded in the market due to such an occurrence, it will trigger at the next available rate.
We do not compensate for these instances as we do not interfere with market conditions or events. Please refer to section 4.4 in our Terms and Conditions for further details.
See also - What is Take Profit?