What is the dew point
Why don't you look up something like this on Google. It is so simple to do.
The dew point is the temperature to which the air must be lowered (the pressure is fixed) for the air to reach 100% relative humidity. That is the point at which dew (or frost or fog) will form on the leaves of plants outside. The dew point is used by meterologists to forecast the minimum temperature for the day or night or for some other period of time.