A stop out is an automatic trigger that will close your order on your behalf, once your available margin falls under 70%. This is triggered so that your account does not end up with a negative balance.
MetaTrader will start off by closing your most negative trade and will continue to do so until your Margin level goes back above 70%.
There are a couple of ways that you can keep your margin levels higher, which is to either increase your equity by adding more margin to your account or alternatively, lower the margin you are trading with by decreasing the lot size of your positions.
Another feature you can use to help reduce the number of stop-outs you encounter is to activate your MetaTrader notifications on your device. By doing so, you will receive a notification whenever a Margin Call is triggered on your trading account.