Sandy is a slave who lives near Mr. Covey's plantation, whom Douglass meets while he's hiding in the woods. Before Douglass fights Covey, Sandy gives him a root and tells him it has magical powers: if Douglass carries the root with him, it will protect him from being whipped. When Douglass wins the fight, it seems like Sandy's root worked, but Douglass isn't sure. Sandy is one of the many slaves who puts his faith in folk beliefs and magic. Douglass mainly just calls this superstition.
In the version of his autobiography that was published in 1881, Douglass said that Sandy was "a genuine African, and had inherited some of the so-called magical powers said to be possessed by the eastern nations."