The dew point is the temperature at which a given parcel of air can no longer hold all its water vapour, so the excess condenses into liquid. It depends on how much moisture the air is carrying: humid air has a high dew point, dry air a low one.
Condensation is, at heart, a comparison between two temperatures — the dew point of the room air and the temperature of the surface it touches. Whenever a wall, window or reveal is colder than the dew point, condensation forms on it. Raising the surface temperature (insulation, heating) or lowering the dew point (ventilation, dehumidification) are the only two ways to stop it.
This is why condensation concentrates on cold spots and thermal bridges, and why it is worse in winter when external walls and glazing are coldest.