It is the heart that God wants us to understand. By understanding the heart, we can understand why we suffer or have joy. As you continue to read the verses, ponder how it is related to the heart, that God wants us to understand.
Interpretation:
God is telling us we must remove the dusts in our minds. These dusts are greed, arrogance, self-love, regret and covetousness. By going on the main path or the path of the heart, we can purify our minds of these dusts.
Heaven's Truths, In Becoming Spirited 