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are the near and far z-values that determine whether the vertex
would be drawn at all.
After the scene is rendered to screen, you see a 2D-representation of 3D points, lines and surfaces, but actually they have 3rd coordinate - the depth
z coordinate). This coordinate defines how far the point is from the observer (the virtual camera). If this coordinate is less than zNear coordinate in GL_PROJECTION matrix - the point is not drawn. The same happens if this coordinate is bigger than zFar.
zNear typically should be bigger than 0. Negative z coordinate means the point is between the image plane and the focal points (where all points meet into one, when z is negative infinity).
The line for z>0 is there just to show the relation of point to other lines, actually ANY point in FRONT of image plane has z>0.