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.
God wants the world to understand the heart. This is why God descended from heaven to save human beings. In verse 1-1 and this verse, God is telling us we do not understand the heart. We can purify our minds when we understand the heart!