Heresy’s spirit of smashing through barriers purifies all of the infection
The verb lifrots means literally "to break down a barrier"; pritsut is the destruction of barriers, depravity. This term usually denotes a rejection of religion, a breaking of religious rules. In this case, however, Rav Kook uses it to suggest a wind that blows through everything and cleanses it. He is speaking here of the fresh spirit of atheism.
The wind of atheism will cleanse the lower levels of faith, to which primitive, simplistic religiosity has become attached, as atheism ridicules and destroys primitive faith. But in so doing, it merely clears the space for the building of deep and more developed faith.
The song of the world is the poetic truth that exists. Rav Kook believes that poetry can convey what is essential, while prose merely describes and systematizes what is revealed through "the song" (for a more detailed discussion see Poetry of Being by Yosef ben Shlomo, Chapter 2, "The General Character of Rabbi Kook's Philosophy.") When the heavens are purified, supernal faith will be revealed and the song of the world and truth of the world will become clearer.