I. In The Metaphysics of Morals ("The Metaphysical Elements of the Theory of Right"), Kant lays out the conditions under which a society is in a state of 'right', a state which, if you care about being in a society (which we all almost always do), he thinks you must accept as the society towards which we ought to strive. He describes it as following:
'Every action which by itself or by its maxim enables the freedom of each individual's will to co-exist with the freedom of everyone else in accordance with a universal law is right.'
Thus if my action or my situation in general can co-exist with the freedom of everyone in accordance with a universal law, anyone who hinders me in either does me an injustice for this hindrance or resistance cannot co-exist with freedom in accordance with universal laws.
The basic idea of this is that: as long as your free actions or your acting on the principles underlying them (the premises from which you reasoned to undertake them) do not interfere with my free actions or my acting on the principles underlying them, then we stand in a relation of right to one another. For example, if I decide to spend the day at the beach, this does not hinder you from doing the same, or from anything else. But if I decide to burn your house down, this would clearly hinder you in both your actions, and situation in general. In the former situation we stand in a relation of right, in the latter we do not. A whole society is in a state of right if all people stand in the former relation, where their free actions, whatever they happen to be, do not impede on the free actions of others, whatever they happen to be. This is the state towards which we ought to strive.