Winston Smith, the protagonist of George Orwell's 1984, is employed at the Ministry of Truth. The function of the ministry is to constantly rewrite all historical documents, including recent newspapers, to make them conform to party dogma and to prevent citizens from having information that might lead them to distrust Big Brother or the Party.
One of the reasons that it is necessary to control the past is that the Party constantly makes promises it cannot keep. For example, the Party promises specified amounts of rations not just of food but also of luxuries such as chocolate or tobacco. Its failure to provide these could stir the citizens to resentment. If historical records were accurate, they could look back and see that the Party is not making accurate predictions and not keeping its promises. When the party changes the past, if people look up information, they will not see the Truth but a fiction that the Party had always promised what it actually delivered. This makes people more likely to doubt their memories than to doubt the party and serves as a tool of indoctrination.
Orwell specifically intended this practice as a criticism of the Soviet Union's efforts to rewrite history books to indoctrinate its citizens. Thus the meaning of the quote is that controlling information about the past gives power in the present, which enables one to determine how things will go in the future.