Data Sovereignty refers to data ownership.
In the present 'freemium' economy most of the Web2 applications have full access and complete control over user data. This includes data created by the user as well as the data about user's application usage patterns and more. Data privacy laws do not enable full transparency or public verifiability regarding how a user's data is accessed and used by the organization that owns the application.
In Web3 the idea of decentralization and dApps is to bring data sovereignty back to where it belongs - the user. With data sovereignty, users have full control over how they store and access their data, who is allowed to access it and every access is recorded through publicly verifiable blockchain backends.