If air is cooled while maintaining the moisture content constant, the relative humidity will rise until it reaches 100%. This temperature, at which the moisture content in the air will saturate the air, is called the dew point.
In basements warm moisture air will diffuse from the warm interior to the exterior walls. As the air passes through the wall, the temperature will drop and eventually reach the outside temperature. Somewhere through the wall assembly the air will reach the saturation point and if filled with moisture will condensate inside the wall, not a good thing, to prevent this we install a vapour barrier or retardant as there is no perfect barrier to prevent moisture movement outward.
To measure the strength of the vapor barrier or retardant we call it Permeance, the lower the Permeance the better the material. The most common is 6-mil polyethylene with a Permeance of 0.06.