Wayne,
Dewpoint, as mentioned above, measures when dew/frost forms based on the humidity. Low dewpoints mean fantastic feeling weather. High dewpoints will feel like a tropical rainforest.
Let me provide a comparison from today:
Philadelphia, PA Temp 95F dewpoint 75F = Heatindex - 107F
Las Vegas, NV Temp 94F, dewpoint 35F = Heatindex 89F
Same air temp, but Philly is not the place to be today. You can also see how a very low dewpoint can actually lower the heatindex. It's mostly because the higher dewpoint results in an atmosphere of higher density.
As an example, you put your hand into an oven of 400F all the time. But if you put your hand into water of 212F, you'll be in the hospital.
The lowest the dewpoint can get in Philly this time of year is probably 55F. It probably averages about 65F (uncomfortable). 70F is muggy. 75F, like today is H-e-double hockey sticks.
www.weather.gov has a great map of dewpoints. If you're looking for great golf weather, look for low dewpoints.