If you are trading on either MT4 or cTrader, Stop out is the event where your position(s) will automatically close by the System once your Margin Level reaches or drops below 50%.
If you are trading on Skilling Trader, then Stop out is the event where your position(s) will automatically close by the System once your Cash Usage reaches or goes beyond 200%.
In case you hold more than one position open, then MT4 will close the most unprofitable one first, while cTrader and Skilling Trader will close the one which holds the highest Margin.
Stop Out exists to prevent a client from losing all available balance of the trading account. It is highly recommended that you closely monitor your account and when your margin level or Cash Usage is close to hitting the Stop out level, you either close some of your positions or fund your account.