This means that Margin Call takes place.
Margin Call is a notification which shows that the little amount of funds is left on the trading account and in case of unfavorable market movement, stop out may take place.
This notification is sent at the moment of time when the remains of funds on the trading account is a certain percent from the margin (for example, 40%).
So in order to avoid stop out you need to deposit or close the positions with floating loss.
You can find the Margin Call and Stop Out % of each account type from here.