Relative humidity is what most of us know as humidity. It is the proportion of water content that the air can hold. The amount of moisture that air can hold before it starts to rain is highly dependent on temperature. To put it simply, the hotter the air is, the bigger sponge it becomes. Even a small temperature change of the air can change relative humidity readings by a few percent. Air at 30°C (94F) can hold twice as much water in it before it starts to rain as can air at 15°C (60F). This leads to the following conclusion: 90% humid air at 15°C, if heated to 30°C, will only show 45% humidity. Since this also works in reverse, we can see that when temperature on a warm and humid day drops due to a weather front coming through, the humidity at lower temperature is likely to reach 100%. This lower temperature, where humidity level reaches 100% is called a dew point, which is reported in most weather forecasts.
Dew point is the temperature, at which moisture in the air reached a point where is starts to condensate. It is a temperature at which we have to cool air for relative humidity to reach 100% level. Without any other influences on the amount of water absorbed in air, this is the temperature at which dew may form and it may also start to rain. This effect can be clearly seen when driving on a cold rainy day. In a warm car the air can hold a certain amount of moisture, but if the glass is cooler than the dew point temperature of the air inside the car, windows will start to fog up. The solution is to warm the windows above the dew point temperature by blowing hot air on them or to turn up the ventilation to reduce air humidity levels inside the car.