The Missouri Compromise was a big deal because Missouri wanted to become a state but as a slave state which caused a lot of issues. The balance between Free and Slave states was equal and adding Missouri would disturb this balance. And made several Northern uneasy so to equal everything out a compromise was made. Missouri could became a slave state and Maine was allowed statehood as a free state. Also in hopes to fix future problems, the Senate came up with the idea of 36' 30" for future states, any states above this shall be a free state and below will be slave states.

Maine-free
Missouri- slave
Balance of Slave vs Free States
36' 30"

Missouri Compromise