Question

what is meant by zenith

Answer 1

the point on the celestial sphere that is directly above the observer.

Answer 2

The zenith is an imaginary point which is situated directly above a particular location on the imaginary celestial sphere.

Above here means the vertical direction which is opposite to the apparent gravitational force at that location.

The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir. The zenith is the "highest" point on the celestial sphere.