Also known as a mangled-address, is a Cardano payment address that contains payment part and staking parts of different wallets/private-keys. This is made possible due to the unique design of addresses on Cardano.
A Franken address allows users to separate their staking rewards into a separate wallet. Some application developers can leverage delegation rights to receive rewards or help users maintain their staking rights while those funds are locked in the smart contract. Additionally, Franken addresses are also used as a way to maintain privacy for assets held in a wallet.