Short answers, pulled from the story.
The Fugitive Slave Clause is Article IV, Section 2, Clause 3 of the U.S. Constitution. It required any person "held to Service or Labour" who fled to another state to be returned to the party claiming their service. The clause notably avoids using the words "slave" or "slavery."
The clause was proposed by Pierce Butler of South Carolina on the 29th of August, 1787, at the Constitutional Convention and passed without debate or objections. It became part of the Constitution when the document was ratified.
Historian Donald Fehrenbacher concluded that the Constitution's framers deliberately avoided the word "slavery" throughout the document to make clear that slavery existed only under state law, not federal law. A last-minute revision changed the wording so the Constitution could not be read as legally sanctioning slavery itself.
Several Northern states passed personal liberty laws to circumvent the clause. Massachusetts prohibited state officials from assisting in fugitive slave renditions and banned the use of state facilities for holding alleged fugitives. These measures were partly a response to the Fugitive Slave Act of 1850.
In Ableman v. Booth (1859), Chief Justice Roger B. Taney reversed the Wisconsin Supreme Court's decision that had freed abolitionist Sherman Booth, who was jailed for aiding an escaped slave. Taney ruled that states could not obstruct federal enforcement of the Fugitive Slave Clause.
The passage of the Thirteenth Amendment abolished slavery "except as a punishment for crime," rendering the Fugitive Slave Clause mostly moot. An earlier effort to repeal the clause directly, in 1864 during the Civil War, had failed.