The mutability of the past is one of the key ideas brought out in "1984".  This means that the past can be changed to reflect the ideas of whomever is in control.  The controlling powers can then make people believe that things happened that didn't really happen, thus giving the illusion that the controlling powers were always right and always good.  Winston Smith's job in the Ministry of Truth in the story was to do just that - rewrite the past.  He would receive little tidbits of old news stories to rewrite so that the stories now reflected what the Party in Oceania wanted people to believe.  The quote you refer to supports this idea of the mutability of the past.  In a power can alter what people believe happened in the past, then they can control people how people think and behave in the future because of these altered perceptions.  In order to alter the written history, a power must be in charge in the current day, thus "...whoever controls the present controls the past."  Oceania constantly changed the past to make themselves look good.  They would change the facts about who they were at war with.  One day. it might be East Asia, the next, Eurasia.  If they were at war with Eurasia, then the Party would rewrite the past to make it say that they had always been at war with them and that East Asia had always been their ally.  The next day, the opposite might appear in historical texts and newspapers if the war had changed fronts.  Oceania might change their predictions, i.e., they might predict that wheat production would be at 60% and when it actually turned out to be 40%, history would be rewritten to show that their prediction had actually been 40% and they were right all along.