An output descriptor is a short, human-readable string that describes exactly which output scripts a wallet owns. For example:
wpkh([d34db33f/84h/0h/0h]xpub.../0/*)#checksum
This says: native SegWit single-key outputs (wpkh), derived from this extended public key, starting from the given fingerprint and path, for every index *.
Why they matter
A seed phrase alone does not tell a wallet which script types and paths to scan. A descriptor does. With descriptors, backups and watch-only wallets become precise and portable between wallets. Standard functions include pkh(), sh(), wpkh(), wsh(), multi() and tr(), defined in BIPs 380–386. The 8-character checksum after # catches typos.