As a surface vibrates it creates nodes, these are places where there are no vibrations.  As shown in the picture below the black rod is vibrating back and forth from the blue dotted line.  The red spots show places where the rod does not move.
 The top boards on guitars also have these nodes (many pictures of these can be seen at: http://www.phys.unsw.edu.au/music/guitar/patterns_engl.html, the black lines are black sand that was sprinkled on the guitar before it was vibrated.  The sand then settled in the nodes.  These pictures show just how variant and complicated the sound in the resonating chamber can be).  These nodes are useful when tuning a finished guitar so  the guitar can be tuned to be most efficient.  By being tuned I'm talking about the guitar itself, not the strings.  When a guitar is tuned it makes the guitar vibrate at the same frequency as the strings so they are not working against each other, this will let all the energy go towards amplifying the sound waves and not fighting against different frequencies.