What is Viscosity and why is it important?

Viscosity is the measure of a liquids resistance to flow,...now why is that important? It's important because the primary function of a lubricant is to keep surfaces from coming into contact with one another.
In engines or equipment an oil achieves this through its viscosity depending on the speed, load and temperature. High speed = low viscosity, Medium Speed and/or Medium Load = medium viscosity, High Load and/or Low Speed = high-viscosity. It is always good practice to compensate for temperature when selecting a suitable viscosity. i.e a gearbox manufacturer recommends using an ISO 150 Gear Oil at normal operating temperature in the planetary gearbox. However, the gearbox is operating inside an industrial freezer at -40°C. In this instance one would have to drastically reduce the lubricants viscosity so that it remains fluid during operation.