The Kansas Nebraska Act repealed the Missouri Compromise. The fight over slavery had been going on since the country began. The Missouri Compromise limited the number of new slave states, but then The Kansas Nebraska Act made it so that settlers in new territories and new states would be allowed to decide for themselves whether they wanted slavery or not. This was called 'popular sovereignty'.
The Republican Party was founded because of this act. Which led, only a few years later, to the Republicans getting a president elected, a tall skinny fellow named Abraham Lincoln. You might have read about him.