This was part of the Compromise of 1850. It stated that if a slave escaped to the north the northerners were required to turn them in. To enforce this southerners would also pay bounty hunters to the north to retrieve their slaves for them.
It required all citizens to report to the police anyone who looked as though they might be a runaway slave, or risk a heavy fine for keeping quiet.
This inflamed Abolitionist opinion, as well as proving unpopular with many Northern citizens generally.
It is an overstatement that it led to the publication of Uncle Tom's Cabin.