The wind chill index describes an equivalent temperature at which the heat loss from exposed flesh would be the same if the wind were near calm. A wind chill index of zero indicates that the effects of wind and temperature on exposed flesh are the same as if the air temperature were zero degrees even though the actual temperature is much higher. The importance of the wind chill index is an indicator of how to dress properly for winter weather. The wind chill does not apply to inanimate objects like your car, or water pipes.
Mittens are better protection than fingered gloves.
To use the chart, find the approximate temperature on the top of the chart. Read down until you are opposite the appropriate wind speed. The number which appears at the intersection of the temperature and wind speed is the wind chill index.