Viscosity is the resistance to flow, so higher viscosity fluids flow more slowly at a given level of force pushing them along. Viscosity has internal friction of fluids, which causes the fluids to appear thicker when flowing. Knowing a fluid's viscosity makes its flow rate predictable under certain circumstances.
Viscosity is affected by temperature, and fluids at higher temperatures tend to flow more easily. Predictions of flow rate are possible when the viscosity and forces known are involved, as long as the fluid is not compressible and the flow is laminar. This means that the molecules of fluid flow in straight lines parallel to the walls of the pipe or other channel. Laminar flow is more likely to occur when the fluid is slow-moving, when it is viscous, or when the pipe is relatively small.
Turbulent flow is the most common type of flow, and it is very chaotic relative to the laminar flow. In turbulent flow, while the molecules of the fluid have an overall movement in the direction of flow, within that flow they move in random and unpredictable directions. This consumes a large amount of the kinetic energy of each molecule, and it makes the actual rate of flow very difficult to predict mathematically.