Capacity factor is a ratio (usually expressed as a percentage) of power actually generated by an installed wind turbine compared to its theoretical maximum power output.
This is important as the available wind varies considerably by location and you want to pick a location that ensures maximum utilization. Although typically the maximum capacity factor achieved in practice is around 30%. This can be compared to the typical capacity factors of 15% for solar and nuclear which ranges in 60 to 100%. Base load thermal power plants (i.e. fire fired) often achieve 70 to 90%.
When capacity factor is combined with cost of production you get the effective cost of power production for a given energy producing technology; i.e. cost divided by capacity factor. This figure you can use as a basis for a ROI (Return on Investment) comparison.