The Missouri Compromise was one of a series of compromises over the expansion of slavery. It is usually identified a key event in causing the American Civil War.
Before 1820, congressmen from the north and the south were very careful when admitting new states to the Union. The number of slave and free state had to always be balanced in order to prevent any one side from gaining too much power. To help this balance, the Ohio River was chosen as the official boundary between free and slave. States were then submitted in slave/free alternating order or in pairs to prevent the balance of power in the Senate from being upset.
In 1820, the state of Missouri applied for statehood. This caused quite an uproar for two reasons. First, it wanted to be a slave state, but most of it lay north of the Ohio River boundary. Second, the state would have upset the balance between slave and free states since no other northern state had applied for statehood.
Northern senators blocked the admission of Missouri while southern senators threatened secession if Missouri wasn’t admitted immediately. Finally, Henry Clay submitted a plan known as the Missouri Compromise in 1820 which solved the issue. Missouri would be submitted as a slave state while Maine, which was part of Massachusetts at the time, would be submitted as a free state. He also proposed a new boundary known as the Missouri Compromise line be drawn as the new slave/free frontier.
The compromise may have allowed Missouri to be submitted, but it would not solve the slavery issue itself.
The Missouri Compromise included that Maine would enter the Union as a free state. Missouri would enter the Union as a free state. The remaining territory of the Louisiana Purchase would be closed to slavery.