What does this Party slogan mean? "Who controls the present controls the past, who controls the past controls the future."

First, the party slogan in George Orwell's book 1984 is actually "Who controls the past controls the future: who controls the present controls the past," not "Who controls the present controls the past, who controls the past controls the future."
This slogan first appears early in the book, in passing. Later, it occurs in a more substantive discussion between O'Brien and Winston. In Winston's perception, the past has "never been altered," despite the fact there is unambiguous evidence that the Party has regularly engaged in historic revisionism. "All that was needed," Winston recalls, "was an unending series of victories over your own memory."
In other words, the slogan posits that actions of today determine the course of events tomorrow. It further posits that actions taken in the present are inspired by events that have occurred in the past. In order to inspire actions designed to advance the ideological cause of the party, therefore, the presentation of the past needs to be done in such a way as to usher the population along the right trajectory.

The slogan means, as O'Brien keeps impressing on Winston during his imprisonment, that power is more important than truth. To put it another way, whoever has power can insist that their version of reality is what passes as the "truth." The Party, having total power in society (or so O'Brien thinks), controls the discourse about the present and the past.
The Party constantly alters the story of the past to suit whatever happens to be its present circumstances. It controls the story of the present because nobody dares contest whatever version of the truth the Party is currently promulgating. Anyone who does so will be arrested by the Thought Police and forced to "disappear" as a warning to others. People learn to quickly believe whatever the Party says.
The last part of the statement, namely, he "who controls the past controls the future," means that, to the Party, truth is just a story and not an objective reality. Whoever controls the story or discourse of the past can make the future bend to their will. Tyrants from time immemorial have believed this idea: it usually does not end well for them because objective reality, despite Party ideology, has a way of interfering with words.

In the dystopian nation of Oceania, the Party rewrites and erases history in the Ministry of Truth in order to make all past documents and records coincide with the Party's current stance and political agenda. By rewriting and altering historical records, the Party appears never to be wrong. Since the Party controls the institutions and ministries that create propaganda, the government is able to alter historical records and essentially "control the past." Therefore, the citizens of Oceania cannot rely on historical records to contain true information because the Party controls the institutions that produce and alter past documents. Since there is no way of effectively countering the Party's stance and exposing them as liars, the citizens are unlikely to attain evidence that would motivate other dissidents to overthrow the ruling Party. Essentially, the Party's ability to erase and alter historical documents gives them the power to control the future.