Blockchains, just by themselves, are a collection of nodes sharing data and agreeing on its state. To make blockchains useful for the end-users, applications are built on top of them. Simply put, applications which use a decentralised backend (like a blockchain), are called Decentralised Applications or DApps.
A simple example of a DApp is software wallet applications that allow the user to query and transfer balance on a blockchain while interacting with a familiar user-interface.
Episode 11 - What are Decentralised Apps (DApps)?