What is the significance of the Mason-Dixon line?

It is the boundary between the slave states and the free states

The Mason-Dixon Line is the boundary between the states of Maryland and Pennsylvania, but it came to represent more than that. It designate the boundary between the northern "free" states and the southern "slave" states in Civil War times. Charles Mason and Jeremiah Dixon set out in 1765 to survey the land and settle a dispute about land grants between Pennsylvania's Penn family and Maryland's Baltimores. The term Mason-Dixon Line was first used in congressional debates leading to the 1820 Missouri Compromise, which determined where slavery would be allowed as the United States expanded.