As water soaks underground, it first passes through a rock layer that has pores that are mostly filled with air. what zone is this called

This zone is called as the zone of aeration. It is present between the earth's surface and the water table, and its main constituents are the soil and rocks. The pores which are present in this zone are partly filled with water, and may mix up with air, causing aeration.

It is called the Zone of Aeration.