For this paragraph:

The kings of Spain, Santiago bishops and nobility paid high taxes and they made large donations of palaces, monasteries and lands a powerful Cluniac order enjoying a true religious and economic monopoly in Spain

We have to explain what this quote means "Cluniac order enjoying a true religious and economic monopoly in Spain"

and I have no idea.. can someone give me some hints, help me out a bit?

It means that the Cluniac order had no competition and ruled the religious and financial life of Spain.