From the Publisher's Website:
The Hidden World is a game of mages and magic set in modern times. White Mages and their aides use any of several styles of magic to defend the world against various kinds of evil.
Basic task resolution is performed by rolling a number of dice equal to the appropriate skill. The highest value die rolled is the value of the roll. If multiple dice have that same value, the dice values are added together. If all dice roll the same value, roll all of the dice again adding to the previous result. Any size dice can be used so long as all rolls are made with the same type of dice.