When you cool air which contains water vapor, the vapor will eventually condense, forming fog or clouds which lead to rain. The "dew point" is the temperature at which this occurs. This temperature is higher when the air contains lots of water vapor, and lower when it contains less.
I've written several Mad Scientist answers about water vapor and temperature: take a look at
By the way, the definition of "dew point" is in my Webster's dictionary. You can get faster answers to your questions by doing some research yourself before you ask the Mad Scientists.
Try the Earth Sciences links in the MadSci Library for more information on Earth Sciences.