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 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.
Fluid is the cheapest component of your power steering system. Changing it can help prolong the life of 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. 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.