whats the one drop rule?
It's based on ideas of white supremacy and the necessary purity of the white race (think of slave owners and later the Ku Klux Klan), so one drop of black blood (a black ancestor) would be enough to make a person be classed as non-white/black. Similar rules applied in Nazi Germany where a Jewish ancestor would make someone non-Aryan and so worthy of a trip to the gas chamber. To a lesser extent it applied in Apartheid South Africa.
Some black people have tried to reclaim it so that the "drop" of black blood purifies the impure non-black blood.
So, an extremely racist concept with very nasty history.