The zone of saturation is where water has infiltrated the ground through the unsaturated zone and becomes saturated. When the layer of the ground becomes saturated this means it cannot hold anymore water droplets.

From the picture to the left you can see the soil in the zone of saturation has been magnified. As you observe you can still see pores not holding water droplets (white). This is because most but not all of the pores will be filled with water in the saturated zone.