Way back in 1819, the U.S. was divided fairly evenly along pro- and anti-slavery states. The scales were balanced—which meant that no one had to address the seriously awful issue of, you know, owning humans.
But as westward territories became more eager to join the Union as official states, this presented a problem to these two factions in American politics. It was a big, geopolitical dun dun dunnnn.
Any additional state on either side would upset the haphazard balance that had so far been struck. This came to a head when Missouri pursued statehood in 1819, forcing legislators to confront the issue of slavery. They really didn't want to do that—because old white dudes felt uncomfortable, or something—so instead a series of bills were drafted and passed in a single block, leaving everyone unsatisfied and thinking they had gotten the short end of the stick.
Especially the slaves, of course, but the aforementioned old white dudes in power didn't really care about them anyway,
In the short-term, the Compromise had one job, and it did it well: preserve the Union. It was a hackjob attempt at stalling on the issue of slavery, and in this it performed admirably. However, as an actual political solution to the problem of slavery, it was woefully inadequate and would become entirely outdated in only thirty years.
The U.S. had faced a number of crises early in its history, but most of these had been external forces (*cough British cough*). The Missouri Compromise represents the first instance in American history when a major crisis from within threatened the integrity of the Union, and U.S. legislators did a remarkable job in avoiding the crisis, or at least delaying it.
Congress desperately needed a solution to the problem of slavery and statehood that adding Missouri posed, and this solution was found through the deft political maneuverings of a dude named Henry Clay. Missouri was established as a state with its own right of self-determination, and an invisible line was drawn through the U.S. defining where slavery could (and could not) be adopted.
The Compromise starts off with a sort of "statement of purpose," opening with the introduction of the Missouri as a state. It then goes through a section-by-section breakdown on the caveats to this introduction. First and foremost is a definition of just where exactly this "Missouri" place even is.
That might seems stupid and needlessly picky, but bear in mind that the single greatest asset America had in these early years was land. Land was used as an incentive and as a bargaining chip in all sorts of early American political and economic ventures, so determining exactly how much of it constituted Missouri was actually super-important.
The Compromise then goes on to breakdown the electoral makeup of the state and to declare that all free white male citizens twenty-one years or older will be able to vote and run for office. Provisions are made for the formation of a seat government (state capitol), as well as a state convention on the formation of a state constitution and other legislative minutiae.
The Compromise then makes further caveats on Missouri's statehood. It sets aside space and funds for schools, roads, and "seminary of learning" i.e. education beyond primary school. Water rights are set down, with provisions. Laws are set down for the parceling of land for military service. Protocol is set down for the acceptance of the state constitution by the Federal government. (Not going to lie: this part is a teensy bit dry.)
Finally, like a footnote, the most important part of the document is mentioned: that the U.S. would be split in two by law, between slave states and free states.
Missouri's a state, and the whole U.S. is now physically divided into slave and free states by law.