A unitary state concentrates governing authority in a single national government, which may delegate administrative power to provinces, regions or local councils but can alter or withdraw that delegation through ordinary national law. That distinguishes it from a federal system, where subnational governments hold constitutionally guaranteed powers the national government cannot simply take back.
Most of the world's states are unitary rather than federal, and the arrangement predates any theory written to justify it: a single crown or central government simply extended its authority outward from a capital as the state itself formed, rather than uniting previously separate, self-governing territories the way a federation does. France is the paradigm case, its highly centralized administrative tradition tracing through the Revolution and Napoleon's reorganization of the country into departments answerable to Paris.
Decentralization within a unitary state is a matter of policy rather than constitutional guarantee, and it varies enormously in practice: a unitary state can devolve extensive day to day authority to local government, as the United Kingdom has done with Scotland and Wales, without ceasing to be unitary, because the central Parliament retains the legal power to reverse that devolution.