Decolonization: The process by which colonies become independent of the colonizing country. It usually involves a transition from being a territory governed by an outside power to being an autonomous nation.
Sovereignty: The authority of a state to govern itself or another state. It's like having full control over your own house - you decide what rules apply within your walls.
Nation-state: A form of political organization under which a relatively homogeneous people inhabit a sovereign state; especially : a state containing one as opposed to several nationalities. Like if everyone in your school belonged to the same club and followed the same rules set by that club.