The statement suggests that having power, or opportunity to take advantage of people, leads to the ill use of such power for personal gain, politically, socially, financially, etc, known as corruption. It implies that possession of it is the cause of such action and it occurs without exception, and that the more power, the more corruption. I would suggest this is because it provides opportunity for personal gain, relative to the amount of power, which becomes harder and harder to resist as the power increases.