The Prophesy spoken of is actually revealed in the Star Wars: The Clone Wars
Series, Season 3, Episodes 15-17. And explanation can be found here:
http://starwars.wikia.com/wiki/Father_(Mortis)
In a nutshell Anakin was to replace Mortis and keep/restore balance in the Force.
But when faced with the choice of remaining as Guardian of the Force
or continue to fight in the Clone Wars, Anakin chose to selfishly continue the war.
Thus bringing about the future we all know from Eps 4-6.
Hope that helped
~ Izzy