Hard edges on models can improve the look of areas which should appear planar, or have problems with triangles becoming visible.  These edges are simply where the normals of the vertices go indifferent directions to define how the lighting should reflect.

Different packages use different methods to define these areas, for example:

Below is an example to illustrate when hard edges are needed.


It should be noted that the more hard edges used in the scene, the slower the renderer will be as it processes every hard edge to anti-alias it.  For this reason you should not over use hard edges, use them where necessary.