The dew point is the temperature at which objects starts to condensate.
Air can hold a certain amount of water in evaporated form. The amount of water that air can hold depends on temperature. The difference between the amount of water that the air can hold and what it actually holds - expressed as a percentage of what it can hold - is called the relative humidity.
If the temperature drops, air can hold less water, and relative humidity will go up. If the relative humidity is 100%, the air is saturated with water, and can not hold any additional water.
As long as there is less water in the air then the amount it can hold at that temperature, condensation will not take place. However, as soon as the temperature drops too low and the air at that temperature can not hold the amount of water that the air actually contains, the water will then form droplets.
If we bring a cold object in a warm environment, the cold surface of this object will cool the air on that surface. If this temperature drops below the dew point, condensation will occur.
The dew point is not only dependent on temperature, but also on relative humidity. If the relative humidity is very low, the air contains little water and the temperature can drop further before the air becomes saturated and droplets will form. This means that the difference between the actual temperature and the dew point is large.
If at the same temperature the relative humidity is high (there is a lot of water in the air), and the temperature can only drop a little bit before the air becomes saturated, then droplets will form. The dew point in this case is much closer to the actual temperature, and only a little drop in temperature can already cause condensation.
The dew point can be determined by using the Mollier diagram or Hx diagram. Programs available for mobile phones, laptops and tablets can simplify the use of this Mollier diagram, as the calculation is already done. More advanced electronic temperature and humidity recorders often have a built-in dew point reading already.