What is meant by dew point?

The dew point is the temperature at which air is saturated with water vapor, which is the gaseous state of water.

When air has reached the dew-point temperature at a particular pressure, the water vapor in the air is in equilibrium with liquid water, meaning water vapor is condensing at the same rate at which liquid water is evaporating.

the apparatus dew point in which condensation or water begins to form at the coils, this is at the saturation point.

It's the temperature when the water vapors present in a volume of air at a specific  atmospheric pressure achieve saturation and condenses to form dew.

The amount of water vapors present in the air makes direct impact on dew point.