When does the air become saturated?
When air cannot hold anymore water vapor molecules it s said to be saturated.
Water on earth evaporates, and it as a form of vapor exists in the air we breath, and the value of how much water there is in air is called humidity.
But air has a maximum capacity to which it can hold water vapor to, and when this capacity is reached and the humidity reaches 100%, then the air is said to be saturated and it reaches the dew point.
Dew point is the point at which air cannot hold anymore vapor molecules, so that some of the water has to condensify and become liquid.