During this time, there were slave states and free states. Slave states allowed slavery, while free states outlawed it. It was important to keep the balance of power with the number of states that were slave states or free states.
The Missouri Compromise of 1820, helped keep the balance of power between the two. The United States senate and other important people, decided that without the balance of power, things would tip and war could even break lose. A lot of people took this very seriously in order to please the United States citizens properly and prevent a war or even other occurrences.
With this compromise, Missouri would become a slave state, but Maine would become a free state. This kept the balance of power between the two, without one getting one more state than the other.