Simply put, cashu is a protocol for digital cash (ecash). It describes how you can an exchange any asset, e.g. bitcoin, for tokens (minting), and later convert the tokens for the underlying asset (melting)

For more information on how it works and additional context, Calle's interview with Odell is a must listen: https://fountain.fm/episode/9F41y47cxqN2LlE4yQKd

Also this old BM article: https://bitcoinmagazine.com/culture/genesis-files-how-david-chaums-ecash-spawned-cypherpunk-dream

And this: https://conduition.io/cryptography/ecash-dlc/#Ecash