The dew point temperature is the temperature at which the air must become cooled to in order to become completely saturated with water vapor. If the air is cooled to the dew point temperature, it will become saturated and condensation will begin to form.
Condensation is the drops of water on the outside of a cold glass of lemonade on a hot day. Because the cold glass had reduced the air surrounding the glass, the temperature of the air has reached the dew point and 100% relative humidity. The dew point temperature is what causes dew to form on the grass in the morning. The morning, just before sunrise is the lowest temperature of the day so it is the time when the dew point temperature is most likely to be reached.
The warmer the air, the more water vapor it can hold. The dew point on a warm and humid day can be fairly high, in the 70s Fahrenheit or in the 20s Celsius. On a dry and cool day, the dew point can be quite low, approaching freezing. If the dew point is below freezing (32°F or 0°C), we instead use the term frost point.
The dew point temperature can never be higher than the air temperature. When the two are equal, the air is holding as much water vapor as it can possibly hold and the air is saturated.
You can use dew point temperatures to help determine how humid the air is. A dew point temperature close to the actual temperature means that the air is quite full of water vapor and thus humid. If the dew point is significantly lower than the air temperature, the air is dry and can still hold much additional water vapor.
Once the air is saturated at is at dew point, condensation forms at this can lead to cloud or fog formation. Sometimes, you will see a layer of misty fog or clouds at point not much higher than the ground, you can be certain that the air at that elevation has reached the dew point and condensation has occurred.