In the novel, the Party has changed what is taught as history to fit their vision of the world and to make themselves look better. The Party controls the present day because they are in charge of the government. So, they have the power to go back and rewrite the history books and control what is being taught to the masses. If they can rewrite history to make themselves look better, that helps them to control the future and to portray themselves in a positive light so that they can stay in control. To tie this into our own world, think about history. Each country is going to have a different interpretation of what happened in a war or event that they will put into their history books and talk about on their news. It is important to remember to look at more than one viewpoint on situations and not just trust one source. Luckily, we have the ability to do this. The people in 1984 would be punished if they tried to find the faults in what the government was telling them.
This Party slogan is first mentioned in Part One, Chapter Three of 1984. To put it into context, Winston is thinking about the Party's past alliance with Eurasia. According to the Party, Oceania and Eurasia have never been allies but Winston has a strong memory of their friendship. However, it does not matter that Winston can remember it: the Party says that it never happened and has rewritten the history books and newspapers accordingly.
The Party is able to rewrite the past through a combination of brainwashing and violence. Because people are unable to speak out, the Party takes control of the both the present and the future. It can continue to manipulate the truth without worrying that anybody will contradict its version of events. If somebody does, they are sent to the Ministry of Love, as happens to Winston in Part Three.