This particular quote serves as an illustration, provided by O'Brien, to better clarify the core motivation which ultimately lies behind this entire dystopian world as envisioned by Orwell. Indeed, note that this illustration is found in part of a much longer interrogation, with O'Brien challenging Winston to answer that very same question concerning the Inner Party's motives. Winston, for his part, assumes that O'Brien would present some sort of consequentialist ends-justify-the-means rationale and answers accordingly. However, O'Brien chides him for this answer. For O'Brien, rather than being a means to some separate end, the wielding of power is actually the end in and of itself. As O'Brien puts it (in another quote from this same chapter):
Power is not a means; it is an end. One does not establish a dictatorship to safeguard a revolution; one makes the revolution in order to establish the dictatorship. The object of persecution is persecution. The object of torture is torture. The object of power is power. (Orwell, 1984, Book 3, chapter 3)
This, as Orwell sees it, is the secret that lies behind all systems of Totalitarianism and oppression. They exist as mechanisms to exert power, for the purpose of dominating the people living under them.
Note here how O'Brien describes and understands power specifically in terms of subjugation. It lies in the breaking and domination of the human spirit. Thus, this image of a boot stomping on a human face serves as a summation of this entire vision as O'Brien presents it, both for the Party itself, but also for all systems of oppression.
While O'Brien is torturing Winston Smith in the Ministry of Love, he explains that the Party is only concerned with possessing complete and utter power over humans. O'Brien begins to elaborate on the Party's vision of the future by telling Winston that there will be no other feelings in Oceania besides fear, rage, triumph, and self-abasement. O'Brien explains to Winston that inflicting pain and humiliation is the definition of genuine power and that the Party's goal is to tear human minds apart before molding them back together. Eventually, the Party hopes to completely abolish all positive feelings and enjoyment from society in order to create a fearfully obedient populace. In the Party's perfect world, all citizens will suffer continuously and devoutly worship Big Brother at all times. O'Brien sums up the Party's vision of the future by telling Winston,
If you want a picture of the future, imagine a boot stamping on a human face—forever.
The image of a boot stamping on a human face illustrates the Party's oppressive, violent reign, where all citizens are helpless and suffer under the omnipotent regime. The boot also emphasizes the Party's complete control over Oceania's vulnerable society and its disregard for humanity.
This is O'Brien's none too subtle way of trying to crush what's left of Winston's spirit. O'Brien knows that in order to break Winston, he's got to get right inside him, deep into the very depths of his soul, for that is where hope lies. If Winston's genuinely going to love Big Brother and show absolute loyalty to the regime, he needs to be disabused of any last remaining vestige of hope he may have that things in Oceania can only get better.
O'Brien's frightening vision of a boot stamping repeatedly on a human face is meant to reinforce the message that there's nowhere else for Winston, or anyone else, to go. Resistance is futile; nothing that Winston can say, do, or think, will ever make the slightest bit of difference. Just as the all-seeing, all-powerful Party controls the past and present, it also controls the future. And there's absolutely nothing that Winston can do about it.
This is O'Brien's—and the Party's—vision of the future as O'Brien describes it to Winston.
In O'Brien's telling, in the future the Party will eradicate love (except for Big Brother), art, literature, and even science, along with all other pleasures. There will be no way to tell beauty from ugliness. The only emotion people will experience is triumph, what O'Brien calls the "thrill of victory" and the "intoxication of power. People will feel triumph too as they "trample a helpless enemy." This is boot stamping the human face—forever.
According to O'Brien, only if people live unnatural lives without material pleasures or the joys of love, family, and companionship—only if they are forced to live in misery against their natural inclinations—does the Party know it has power over them. O'Brien explains to Winston that the only power a person can be sure of is the power to make another person do something he doesn't want to do.
As nobody wants to have their faced stomped on by a booted figure, that becomes O'Brien's symbol of power. It represents the total triumph of the hate-filled victor and the total powerlessness of the helpless victim.