A Stop Out Level is when your Margin Level falls to a specific percentage (%) level in which one or all of your open positions are closed automatically (“liquidated”).
This liquidation happens because the trading account can no longer support the open positions due to a lack of margin.
More specifically, the Stop Out Level is when the Equity is lower than a specific percentage of your Used Margin.
If this level is reached, your positions will automatically start closing out with the most unprofitable one until your Margin Level is back above the Stop Out Level.
If your Margin Level is at or below the Stop Out Level, positions will close quickly as possible in order to protect you from possibly incurring further losses.
This act of closing your positions is called a Stop Out.
Keep in mind that a Stop Out is not discretionary. Once the liquidation process has started, it is usually not possible to stop it since the process is automated.