Martin Luther King Jr. understood that we are all brothers and sisters, we are all in life together. He also understood that our sin lay at the root of all our injustices toward each other.
In this speech he speaks to the suffering, failing and hardships that we experience. If we stop to contemplate and understand, we truly learn something useful, or even gain insight in some way.
The analogy he draws on is that darkness is oppression/hardship, and most of us have some form of hardships/adversity that torment us. The stars in heaven is the wisdom and experience we can develop as a person (in a positive way) from the suffering, and grow stronger as a result.
“It really boils down to this: that all life is interrelated. We are all caught in an inescapable network of mutuality, tied together into a single garment of destiny. Whatever affects one directly, affects all indirectly. We are made to live together because of the interrelated structure of reality . . . Before you finish eating breakfast in the morning, you’ve depended on more than half the world. This is the way our universe is structured, this is its interrelated quality. We aren’t going to have peace on Earth until we recognize the basic fact of the interrelated structure of all reality.”
-Dr. Martin Luther King, Jr. 1967 Christmas Sermon on Peace