“Destiny” refers to the unseen results of ones past activities that have begun to fructify. It is supreme in the sense that it leads a man to good or bad [experiences he deserves]. “Human endeavor” refers to the execution of dhārmika activities in this life. They are unproductive in the sense that they are incapable of neutralizing the results of his past activities that have begun to fructify. Destiny is the ultimate shelter in the sense that it awards one happiness and unhappiness in this life by without considering the [inauspicious and auspicious activities] performed in this life.

NOTE. Śrīla Prabhupāda sheds some light on the unseen results of one’s past activities that have begun to fructify in his Introduction to Bhagavad-gītā:

“The process of quitting this body and getting another body in the material world is also organized. A man dies after it has been decided what form of body he will have in the next life. Higher authorities, not the living entity himself, make this decision. According to our activities in this life, we either rise or sink. This life is a preparation for the next life.”