In engaging in practical endeavors (deciding what to do, holding oneself and others responsible), one is justified in holding oneself to all the principles that would bind an autonomousfree will.
(contrasted with purely theoretical or abstract thinking)
Real-world activities and decisions you actually make in life, like choosing a job, keeping promises, or deciding how to treat others.
justified(Epistemological discussion of Socratic wisdom in The Apology)
Having beliefs formed with adequate evidence or through reliable belief-forming processes, distinct from the ability to demonstrate one's justification to an interrogator.
principles(Explicitly equated with 'invariant reasons' in the passage)
Invariant reasons — moral considerations that apply consistently regardless of particular circumstances
Kant says that a will that cannot exercise itself except under the Idea of its freedom is free from a practical point of view (im practischer Absicht). In saying such wills are free from a practical point of view, he is saying that in engaging in practical endeavors — trying to decide what to do, what to hold oneself and others responsible for, and so on — one is justified in holding oneself to all of the principles to which one would be justified in holding wills that are autonomous free wills.