Basically, if you trade 500$ and lose it all, they refund it in your trading account. Here are more details I found on their website:

"What is a protected account?

Trade on forex, commodities, stocks, indices and bonds for 30 days without the chance of losing! Everything you make is yours, and your losses are on us. It's real money and real profits without the risk of losing. Protected accounts are one of the most impressive features offered by 4XP.

This unique service provides traders with an opportunity to experiment with their preferred strategy on an award winning trading platform- the MetaTrader 4 or the MetaTrader 5. Learn, practice and trade on stocks, bonds, indices, commodities and forex 100% risk free, 100% beneficial and 100% live- all the profits are yours, all the losses are ours. A protected account offers a full 30-day risk free account that cannot lose- only profit!

By taking advantage of 4XP's latest offer, you will become an expert at trading in the Capital Market. 4XP will teach you everything you need to become a professional trader.
Learn, Practice and Trade 100% RISK FREE!"