Let's say you go long (buy) the DE30 (based on the DAX underlying) and adopt a risky strategy of utilising the majority of your account balance in this one trade as margin. This means that any large move in the market could have a big effect on your account funds.
On Friday the market is trading slightly above your entry point, meaning you're in profit as the market closes for the week. However, over the weekend there's some political turmoil in Germany causing investor uncertainty which means that when the DE30 re-opens on Monday, its opening price is significantly lower than Friday’s close, resulting in a large loss that exceeds all the funds on your account, and putting you into a negative balance.
With negative balance protection however, you will not end up owing us this negative amount as we'll write it off for you. So when you trade with XTB, your maximum loss on any trade is the funds in your account, regardless of what the market does.