This topic mainly relates to condensation that builds within your wall due to the variance of external temperatures vs interior temperatures. Think back to what you learned about how clouds form in fifth grade earth science. If moisture is present, the variation in temperature between cold and warm air masses will create an environment that increases moisture, which in turn become clouds (way over-simplified, but you get the gist). 

The synopsis presented below comes from Weather.gov and explains how the cloud dynamics perform in the atmosphere, but I present the information here, since it is identical to what can happen in a home’s wall system.  

Dew point is a term describing the temperature at which air at a constant pressure will become saturated and achieve 100% relative humidity (RH). At this point, the air cannot hold more water in the gas form. If the air were to be cooled even more, water vapor would have to come out of the atmosphere in the liquid form, usually as fog or precipitation.

The higher the dew point rises, the greater the amount of moisture in the air. This directly affects how “comfortable” it will feel outside. Many times, relative humidity can be misleading. For example, a temperature of 30 and a dew point of 30 will give you a relative humidity of 100%, while a temperature of 80 and a dew point of 60 produces a relative humidity of 50%. It would feel much more “humid” on the 80-degree day with 50% relative humidity than on the 30-degree day with a 100% relative humidity. This is because of the higher dew point.

See definition: Dewpoint and Humidity.