It was surveyed between 1763 and 1767 by Charles Mason and Jeremiah Dixon over a border dispute between British colonies in Colonial America.
It formed part of the borders of Pennsylvania, Maryland, Delaware, and Virginia (now West Virginia). By the early 1800 is became know as the dividing point between North Eastern and Southern American states.
Originally, a "Crown" stone was placed every 5 miles that bore the family crest of the founders of the states they bordered. Regular markers were placed every mile.
It's often tied into a boundary line of "Free States" and states that still allowed slavery.
Even though Pennsylvania ended slavery in 1781, Delaware held onto slaves until the 13th amendment was passed in 1865.
Today it's referenced mostly by people in southern states who just can't seem to let go of the past, and have this belief that the "South" will once again rise.