It was a secret passage for slaves to escape the Southern United States.
The Underground Railroad was a secret route developed during the early to the mid-nineteenth century to help slaves on their escape to freedom. Basically, it was a term that describes the network meeting places, secret rooms, and secret houses that were used by slaves in the U.S. to escape. There are about 100,000 slaves escaped in the South between 1810 and 1850.
The name was given underground because it gave the way of escaping to prisoners and slaves. Underground referred to the word secrecy, and railroad denoted to transport the people.