Until now, Bitcoin Core was designed to manage the UTXOs in its wallet around their corresponding private keys — even though private keys are just one of several potential conditions for spending coins. Bitcoin Core 0.21.0 instead introduces “descriptor wallets.” Descriptor wallets let users categorize their UTXOs based on the types of conditions that are required to spend them. (For example: one wallet for UTXOs that just require a valid signature, and one wallet for multisig UTXOs.)
Descriptor wallets are especially useful for application developers who design software on top of Bitcoin Core. A particular application can now easily be designed to utilize only a specific type of UTXO, like multisig UTXOs, and ignore any non-multisig UTXOs.