Those are heating degree days. They are calculated using a starting point of 65F, assuming that an average house doesnt need any heat above that temperature. then you just add up the degrees below that point for each day of the month. For example if on day one the average temp for the day is 50, thats 65 - 50 = 15 HDD. And on day 2, the average temp for the day is 40, that's 25 HDD And on day 3 the average temp is 60, that's 5 HDD. You sum those figures up over whatever time period you want.