Answer:
In this poem, Beaty takes the persona of a little boy whose father was present in his life, but who then was imprisoned. The little boy doesn't understand prison-- he wants to reach out to his father but cannot, and as the years pass, his father is no longer a factor, no longer there to mentor and guide him as he grows to adulthood. This is painful, since he once wanted to be like his father but now can barely remember him.
But rather than losing hope, the boy (who is now a young man) decides to use his ability to write poetry as a way to heal. He imagines his father has written him a letter, answering his questions. And since his dad is not available to him, he decides to father and mentor himself. Beaty wants his words to inspire and encourage others in the black community, reminding them that they do not have to make the same choices their parent made, that they don't have to be imprisoned (either in the real sense, or in the metaphorical sense-- sometimes, the world can seem like a prison, or a person can feel chained to memories of the past).
Beaty, who is a successful actor, author, and singer, also wants his listeners to know that they can make a positive difference. The refrain of "knock knock" is first about the child waiting to hear his father as they play a game, but later it refers to knocking down the obstacles-- like poverty or racism-- and knocking down the barriers that keep you from changing the world.