The z-axis measures the depth of an object in a 3D world. Each object is composed of a group of vertices (points) that form a polygons which in turn combine to form a complex (usually) object. Assuming that the x-axis and the y-axis define the plane in which the screen (the front of your monitor) is included, the z-axis measures the distance from a point to the screen. In a 2D image the polygons would just be flat, but in a 3D image the polygons actually have depth enabling some polygons to be located behind others either partially or completely. It is this depth that makes a 3D world come alive.