The Fugitive Slave Law became a law in 1793, but was not enforced in the Nothern states. Senator Henry Clay suggested that part of the Compromise of 1850 should include the enforcement of the Fugitive Slave Act.
This was probably the only way California could have been admitted as a free state and the citizens of New Mexico and Utah would be able to decide whether they would be admitted as free or slave states.
The Fugitive Slave Law was passed to help the southern slave owners recapture and punish runaway slave. It forced the Northerners to return runaways.
This angered the Northerners, who became more active in the Underground Railroad.