Decentralized applications (DApps) are applications similar to traditional apps, with the fundamental difference that instead of relying on centralized servers, they run on a decentralized peer-to-peer blockchain network.
DApps provide many benefits, including privacy in data management: a traditional app requires user data to access the services (that are then managed in a centralized database in the hands of the app developer), in the DApps the user uses their own blockchain "account", that is their cryptographic keys, thus avoiding to disclose their data.