Answer:
When there's dispute among states with no other federal court to rule over it.
Step-by-step explanation:
Original jurisdiction means the Supreme Court can hear a case that's come to it directly, without the matter having gone through rulings and appeals in a lower court. This can involve a dispute between states, with no other federal court having jurisdiction over the case. Those matters, however, are pretty rare.
One significant example is New Jersey v. New York, a case from the court's 1998 term. The Garden State sued the Empire State in the Supreme Court over jurisdiction of Ellis Island's artificially expanded landmass, an issue dating to the 19th century.