Okay kiddo, let's talk about something called the Missouri Compromise.
A long time ago, in the United States, there were some places where people could own other people as slaves. Slavery is bad and was eventually made illegal, but in the early 1800s, it was still allowed in some states.
One of the states, called Missouri, wanted to join the United States as a slave state. But there were already a lot of free states (where slavery was not allowed), and people were worried that there would be an imbalance between the number of free states and slave states.
So, some very important people got together and made a deal. They said that Missouri could join as a slave state, but another state, called Maine, would also join as a free state, to keep the balance.
They also made a line on a map, called the 36°30′ line, that said any future states above that line would be free states and any below it would be slave states. This was called the Missouri Compromise.
The Missouri Compromise helped keep peace between the free states and slave states for a while, but eventually the issue of slavery caused more conflict and led to the Civil War, which ended slavery once and for all.