how to fix an error like this

08/08/2009 19:49 Pete1990#1
Ok now and then i see this error and i dont know what to do about it
HTML Code:
System.NullReferenceException: Object reference not set to an instance of an object.
What does it mean how can i fix them?
08/08/2009 21:57 CoAttack#2
you probably went too much on your array i think something about it.
08/08/2009 22:18 samehvan#3
that means u r trying to do an operation on a null variable ,too many cases may coz that so it depends on ur code line