They were intelligent and determined individuals who not only managed to escape to freedom themselves, but worked for the freedom of other slaves.
Details:
When Frederick Douglass was a slave in Maryland, he taught himself to read and write. Then he undertook to teach other slaves those skills. That got him sent to a slave master who was known for his harsh treatment of slaves, at the age of 16. In 1838, when he was about 20, he managed to escape from slavery and went to New York City. He then married a free black woman from Baltimore, and they settled in Massachusetts. He became a prominent and powerful spokesman of the abolitionist movement, seeking to end slavery in the United States.
Harriet Tubman was also an escaped slave. who had been held in slavery in Maryland. She escaped to Philadelphia in 1849 ... but then went back to rescue her family and help them to freedom. Then she helped extended family members, and eventually was helping many, many slaves to freedom on what became known as the "Underground Railroad" -- a secret network of routes from one safe house to another that aided slaves in their escape.