What is the Missouri Compromise?

In the simplest of forms the Missouri compromise was an agreement for Missouri to join the Unions. At that time states were divided on the issue of slavery. There was an even split of pro and against slaver states. If Missouri joined as a slavery state it was thought that the balance would change. To remedy this two states were allowed to join, one for and one against slavery. The states were pro slavery Missouri and against slavery Maine. Thus the balance was kept.