What is a z-coordinate?
You are probably familiar with the x-y Cartesian Coordinate system. This is a 2-dimensional flat plane and locations on the plane can be identified by an (x,y) Coordinate.
Z-Coordinates are found in a geometric setting called three-dimensional space. Unlike the 2d flat xy plane, to identify a location in 3d space, three parameters (variables/values) are required to determine a position/location