Condensation is the process whereby water vapour changes into water droplets. It is an important concept for building surveys because it can lead to damp.
If there is water vapour within a building, it may condense into liquid water. This is more likely to happen on cold surfaces such as walls. Likewise, poorly ventilated areas will experience greater humidity (more water vapour) and the chance of Condensation will, again, be higher.
For these reasons, Condensation often emerges repetitively in the same place. Areas that suffer from repeat Condensation will become damp. If timber becomes damp it can weaken drastically and become a breeding ground for wet rot.
It is important to contact a quality surveyor to assess and treat any Condensation you find in a building.