Answer:
The Missouri compromise addressed which states would allow slavery and which would not.
Initial Issue
The Missouri compromise was initially made to solve one issue. The population of Missouri had become high enough that they could apply for statehood, but Missouri had slavery. At this time, the number of slave states and free states was equal. This meant that the senate was balanced and one side could not overpower the other. Had Missouri been admitted without the compromise, the balance would have been thrown off. Free states in the North were unwilling to cede political power in the Senate, so both sides came to an impasse.
Admission of Missouri and Maine
A man named Henry Clay, also known as the Great Compromiser, created a two-part compromise. The first part of the admission of both Missouri (a slave state) and Maine (a free state). Maine did not technically have enough people to constitute a state, but this allowed the balance to be maintained. With the admission of both states, there would still be an equal number of free and slave state senators.
The 36° 30'
The second part of the MO compromise was drawing a line through the country. Clay proposed that anywhere north of the 36th parallel in the Louisiana territory would be a free state and anywhere south would be a slave state. This line was "drawn" at 36° 30', hence the name. Later, this line would prove to be insufficient at settling slavery debates and would be removed.
Significance
During the 1800s, slavery was arguably the most debated topic in the United States. Making this compromise helped ease the immediate dangers of upsetting the balance in the Senate. However, it did not truly deal with the issue of slavery. So, while the MO compromise helped delay the civil war, it did not decrease tensions between abolitionists and slave owners.