What does the following Party slogan mean: "Who controls the past controls the future. Who controls the present controls the past?"

This Party slogan
is first mentioned in Part One, Chapter Three of . To put it into
context,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.