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:
The Original mind signifies the pure mind, resulting from understanding the heart. When the mind is purified, God tells us to perform the Joyous Service to save others.
OFUDESAKI-GOD'S TEACHINGS 