A degree day is a unit of measure for recording how hot or how cold it has been over a 24-hour period. The number of degree days applied to any particular day of the week is determined by calculating the mean temperature for the day and then comparing the mean temperature to a base value of 65 degrees F. The mean temperature is calculated by adding together the high for the day and the low for the day, and then dividing the result by 2. If the mean temperature for the day is, say, 5 degrees higher than 65, then there have been 5 cooling degree days.
On the other hand, if the weather has been cool, and the mean temperature is, say, 55 degrees, then there have been 10 heating degree days (65 minus 55 equals 10).
Why do we want or need to know the number of degree days?
It is a good way to generally keep track of how much demand there has been for energy needed for either heating or cooling buildings. The cooler (warmer) the weather, the larger the number of heating (cooling) degree days... and the larger the number of heating (cooling) degree days, the heavier the demand for energy needed to heat (cool) buildings.
National Weather Service