I'm having trouble understanding this line from my book:

In perspective projection, the transformed depth coordinate (like the x and y coordinates) is subject to perspective division by the w coordinate. As the transformed depth coordinate moves farther away from the near clipping plane, its location becomes increasingly less precise. (See Figure 3-18.)

Figure 3-18 : Perspective Projection and Transformed Depth Coordinates

Therefore, perspective division affects the accuracy of operations which rely upon the transformed depth coordinate, especially depth-buffering, which is used for hidden surface removal.

first I'm wondering, is the depth coordinate the same as the z-coordinate you originally specify in GLvertex? Second, why does the accuracy decrease? Whats causing that to happen?