Negative balance protection is an important feature that MTrading offers for the safety of its clients. It means that a trader can never lose more money than is in his trading account.
In practice, this is how it works: if the market experiences a sharp price movement and the deal goes against the trader, the system automatically closes positions so that the account balance does not become negative. For example, even if the market moves very quickly and a so-called "gap" (difference between prices) occurs, the trader's losses will be limited only to the amount that was already in his account https://mtrading.com/promotions/negative-balance
This mechanism is especially important for beginners and those who use leverage, as it removes the risk of getting into debt to the broker.