The ambient air always contains water vapour. The warmer the air is, the more water vapour it can hold. At 0°C, air can only hold 4.84 g/m3 but at 50°C it can hold 83.0 g/m3. In both cases the air is saturated.
The Relative Humidity of the air, RH, is the proportional relationship between the momentary water vapour content at a certain temperature and the maximum water vapour content that the air can hold at the same temperature. With a RH of 50 %, air contains only half of the water vapour quantity, which could be maximally contained at the appropriate temperature. With 100 % RH, the air is completely saturated with water vapour. RH can never exceed 100% because the surplus humidity settles as condensation and/or fog. With sinking temperature the capacity to hold water vapour decreases, and the excess water vapour will fall out as liquid water - The temperature has reached the Dew Point and condensation occurs. If air comes in contact with a surface with temperature below the dewpoint of the ambient air, water vapour will condense on that surface.
In a building there are several surfaces that can have temperatures below dewpoint - e.g. outer walls and cold water pipes. The condensation on those surfaces can cause severe damages to the building and its inhabitants - mould, rust, rottening, deformation of materials etc.
To calculate the dewpoint please go to Dewpoint Calculations.