The Gist: In the slavery days, the federal law and U.S. Constitution protected a slave owner's right to recover a slave who had fled to a free state. Pennsylvania state law protected slaves from being recaptured by violence or force. The Supreme Court ruled that the Pennsylvania state law was unconstitutional due to its conflict with federal law.
The Details: At the time, Article IV, Section 2 of the U.S. Constitution (The Fugitive Slave Clause) held that if a slave escaped to a free state, that did not mean the slave was free, but had to be returned to his or her owner. Additionally, the federal Fugitive Slave Act of 1793 required judges to return slaves to his or her owner. A Pennsylvania state law rendered it a crime for a slave owner or agent to recover his or her slave in the state by violence or force. In this case, Edward Prigg was convicted under Pennsylvania law when he forcefully seized escaped slave Margaret Morgan and her family back into slavery. Prigg brought his case to the Supreme Court, arguing that the Pennsylvania law and his conviction was unconstitutional.
The Supreme Court agreed, stating the state law was unconstitutional because the Constitution gave the federal government exclusive power to to legislate fugitive slave. The court did not have to go this route. It could have ruled that the state law was constitutional because it still allowed slave owners to recover slaves via the legal system and that the prohibition of "force or violence" was not in conflict with federal law. To keep peace between the Northern and Southern States, however--to keep a very fragile Union at the time together--the Court decided to broadly protect the rights of the slave owners.
Interestingly enough, however, the Court did say that state legislatures could pass laws to prohibit state officials from enforcing federal fugitive slave laws--requiring federal officials to do so instead--and that's exactly what free states ended up doing. With a lack of federal officials to regulate the federal laws in all of the northern states, this made the federal fugitive slave laws incredibly difficult to enforce, bringing further hostility and separation between the North and South.