Essentially the dew point is the temperature at which a parcel of air can no longer hold all the water vapour in the air and some of the water has to condense out which can form fog/mist.
So if the air temp is higher than the dew point then the air can hold all the water vapour in the air parcel,and it is clear, if the air temp is lower than the dew point vapour condenses out and becomes visible moisture (fog)