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, think of how it is related to the heart, that God wants us to understand.
The worldly path is where one thinks that one can get away with any evil actions, as long as no one sees him or her doing it; or does not care if any one sees it. As oppose to the path of the heart, where one’s action come back to oneself. We reap what we sow.