This song is about one of the most famous "conductors" of the "Underground Railroad"
The Underground Railroad was a network of clandestine routes by which African slaves in the United States attempted to escape to free states, or as far north as Canada, with the aid of abolitionists. Other routes led to Mexico or overseas.
It is estimated that at its height between 1810 and 1850, between 30,000 and 100,000 people escaped enslavement via the Underground Railroad, though US Census figures only account for 6000.
See Wikipedia for more info on The Underground Railway and Harriet Tubman