Power-steering is what enables you to turn your steering wheel, without a great deal of effort. Power-steering fluid, is the hydraulic fluid in the system that transmits the power. If you're having trouble turning the steering wheel or hear high-pitched noises when you turn it, chances are your power-steering fluid is low or may have sprung a leak.
The fluid is the cheapest component of your power-steering system. And changing it can help to prolong the life of other, more expensive power-steering components. If ignored long enough, the power steering pump and/or power steering rack may need to be replaced.
From a safety standpoint, there is a large safety risk, because a failing power steering system will make it difficult to control your vehicle. From an environmental point of view, leaking power steering fluid can be dangerous to children and animals.