Nicolaus Copernicus was probably one of few, who knew what Zenith could be used for in his days, nowadays we naturally know what is means and do not use old tradition navigation instruments any longer, but the knowledge of the Zenith is still very important for orientation.
Zenith is the opposite of Nadir, and the line is always vertical and at a right angle, i.e. 90° to your Horizon.
It is used in concurrence with the Horizon when navigating the Celestial bodies, to determine your position on the earth sphere.