Most people thing there is something morally wrong with the existence of poverty, to the extent that those who are in poverty - or at least, the government which represents them - is entitled to forcibly extract resources from other people to end, reduce, or ameliorate poverty. This is what is meant by "social justice".
Views of this kind are often described as "egalitarian", but in fact one of the most plausible such views has nothing at all to do with equality. Sufficientarianism is the view according to which there exists a level which is "enough" for people; people below this line are entitled to the resources which bring them up to it, while those above are obliged to provide. Sufficientarianism has a lot of intuitive appeal: it is easy to see how a starving beggar might be entitled to the charity of a billionaire, but it is much harder to see how a comfortable homeowner, who while hardly a billionaire has no concern about where his next meal is coming from, would be entitled to this charity. We might still think a world in which the homeowner and the billionaire were more equal would be better, but this falls quite short of implying that the homeowner or his government has the right to forcibly redistribute from the billionaire to the homeowner.
Similarly we might think that the higher one lies above the line of sufficiency, the greater is one's obligation to bring others above the line; but again, this does not require one to take equality as any kind of fundamental value.
One consequence of sufficientarianism, often considered counterintuitive and sometimes considered damning, is what it implies in a world of people who are all or mostly below the line of sufficiency. If the measure of a society is the extent to which it brings people above this line, this seems to imply that we should worsen the lives of some of those who are already below the line in order to bring some others above the line. In extremis, with a world of 100 people narrowly below the line, sufficientarianism may require us to utterly ruin the lives of 99 of these people in order to marginally the life of the 100th so that she reaches the line.
There are of course ways to avoid this conclusion, but I sometimes think we are too quick to reject it. Suppose 100 people are caught in a prison camp, and all would rather die than continue to endure this miserable existence. To wit, they hatch an audacious escape plan which will enable a small number of their fellows to reach freedom. Those left behind will be heavily punished and tortured for their roles in the plot, so the plan could hardly be less egalitarian - yet it is still worthwhile going through with, and it is worthwhile for those left behind to suffer for their fellows.
Is there a clear historical example of this? Indeed there is, and for much of history it dominated our planet. The idea that most people could live good lives is a distinctly modern one, a product of the industrial revolution. Before that, poverty, starvation, and abject misery were the norm and indeed the only possibility 99% of the world's population. Simultaneously, however, there existed classes of knights who enjoyed lives vastly greater than any villein or serf could have hoped for: eating well (by the standards of the time), enjoying education (such as there was), and without having to engage in backbreaking labour in the fields.
It is my contention that from a sufficientarian perspective, such arrangements made perfect sense: almost everyone below what should be considered an acceptable level of wellbeing, but by the sacrifice of the many a few were enabled to live genuinely worthwhile lives.
In the modern world, with abundant food and water, with indoor plumbing and heating, it is hardly necessary to impoverish the masses in order to create lives worth living. But in the complacent post-scarcity society, it is easy to lose sight of the kind of sacrifices which were necessary for our ancestors. Feudalism was not a system of brutal oppression; rather it stands as the greatest monument to the nobility of the human spirit: the willingness to sacrifice oneself for the creation of lives which are truly worthwhile.