James Baldwin's letter to his fifteen-year-old nephew is also known as "My Dungeon Shook." The title derives from its final line of quoted poetry:
The very time I thought I was lost, my dungeon shook and my chains fell off.
Baldwin wrote the letter in 1963 to his nephew, who had the same name, to mark the centennial of the Emancipation Proclamation. It is a deep critique of the dominant perspectives in U.S. society that continued to restrict and oppress Black Americans in his day.
He urges him to remember that those limits speak of the fear of whites, not of black people themselves. The strength he can draw from his people's love will help him to make his presence known in a country that would try to ignore him. He discusses the courage needed to free himself and do his utmost.
You can also find a longer summary in the eNotes Study Guide.