Complex sentence: At the same time, a man shows up who had reserved poolside sun-loungers by putting towels on them, and is angry because the boys have thrown the towels in the pool and taken the chairs for themselves.
Split into simpler sentences: At the same time, a man shows up who had reserved poolside sun-loungers by putting towels on them. He is angry because the boys have thrown the towels in the pool and taken the chairs for themselves.