States' Rights: This refers to political powers reserved for U.S. state governments rather than federal government according to Constitution. It's like your parents letting you choose what clothes you want to wear instead of deciding for you.
Secession: This is the withdrawal of a group from a larger entity, especially a political entity (a country), but also any organization, union or military alliance. Imagine if one team member decided to quit because they didn't agree with team rules.
Slavery: A system where individuals known as slaves are treated as property and forced to work. Slaves can be held against their will from their time of capture, purchase or birth, and deprived of right to leave, refuse work or demand compensation. Like being forced to do all household chores without any say in it or reward.