XVII. The election of some being supposed, the preterition of others follows. By this he [God] not only was unwilling to confirm them in good, but decreed to permit their sin. The fall taking place, he decreed to leave them in and condemn them on account of their sin. Their reprobation to this is referred. It is also contained in two acts: one negative (dereliction in the fall); the other affirmative (damnation to eternal punishment). ... [Francis Turretin, Institutes of Elenctic Theology. 184.108.40.206]
What is reprobation? Reprobation is God's active decree to pass by some (all those whom He did not elect unto salvation) and decree them to eternal damnation. Reprobation, like election, is unconditional. As it condemns people to hell even before they were created in time, it seems to imply injustice in God. That is why Paul had to write Romans 9:14-24, as reprobation seems to be unjust. After all, how can God punish someone before they have even done good or bad?
When one looks more closely at the doctrine of reprobation however, it can be seen that at least some of the concern is misplaced. Reprobation is made up of two parts: Pretertion and Damnation. In preterition, God passed over those whom He did not elect. In damnation, God condemns to hell reprobate sinners because of their sin. Thus reprobation is not conditioned upon the sinner, but at the same time, sinners will never be condemned to hell apart from their sins. It is hypothetically possible for a creature to be passed over (preterition) but not condemned if that creature did not sin, and thus not be sent to hell. It is however not actually possible for any human to only be passed over without being damned, because all Man have fallen in Adam and thus all Man would have and will sin. Thus, God's decree of reprobation will have worked out in preterition and damnation for all of the reprobate, all without exception, and yet there would certainly be no injustice with God.