The one drop rule was the idea that even one black ancestor classified a person as black. The idea that even one drop of black blood was prevalent in social and legal classifications. The one drop rule was a way to further racism and separate black people from everyone else, particularly from white people.
Wikipedia
One-drop rule
Racial Integrity Act of 1924
Video
One Drop Rule Explained (UntoldTruthMedia)
Further Reading
Who is Black? One Nation’s Definition (PBS Frontline)
Racial Integrity Act of 1924
Historical Origins of the One-Drop Racial Rule in the United States (Winthrop D. Jordan)
The One-Drop Rule in American History (Karin Gonzalez – study.com)
Origin & Meaning of The One Drop Rule (solarey.net)