What does Macintyre mean by justice as virtue?

To understand justice as a virtue, one must first understand what exactly justice and virtue consist of. One way to see justice is that it is, as Justinian wrote in The Institutes of Justinian, “giving every man his due.” A virtue is any extremely morally good behavior or trait.
The concept of justice as a virtue dates back to at least the time of Plato in the Republic, in which justice is seen as a moral virtue in both individuals and society at large. He viewed justice as one of the Four Cardinal Virtues; it was absolutely necessary for the stable existence of any good society. Justice, then, would be a moral imperative, a necessary trait of any good person.
Most would likely agree with the general ideas up to this point. The devil is in the details, and that is where MacIntyre's focus lies. Three Rival Versions of Moral Enquiry focuses heavily on the differences that different branches of philosophy took in determining what exactly constitutes "good" and what "each man's due" really is.
https://plato.stanford.edu/entries/justice-virtue/