I assume that you are asking this in the context of the field of International Relations (because of other questions you have asked). In international relations, the idea of state sovereignty is the idea that the state (the government of the country) has complete authority over what it does within its territory.
The idea of the sovereign state is one of the bases of modern international relations. It assumes that, for example, the United States cannot simply tell China to free Tibet or to stop cracking down on internal dissent. Because China is a sovereign state, it has the right to do whatever it wants within its borders.
State sovereignty, then, is the idea that each state is sovereign within its borders and can therefore do what it wants in terms of its domestic policies.