Answer: It kept the balance of power in the Senate equal between the slave states and free states.
Further detail:
The Missouri Compromise (1820) admitted Missouri into the Union as a slave state with Maine being added at the same time to keep the balance of slave and free states equal. It also prohibited any future slave states north of the latitude line 36 1/2 degrees north of the equator in territories of the Louisiana Purchase, with the exception of Missouri (north of that line) being admitted as a slave state.