Let's learn about Cashu:

From their own documentation, Cashu is an Chaumian ecash system built for Bitcon.

Chaumian refers to a technique called Chaumian Blinding. In short this technique allows a transaction to be carried out without revealing the parties involved on either side of the transaction.

Cashu is a protocol. Kind of like how Nostr is a protocol, or Bitcoin. This way anyone can develop tools that can talk to any other tool that implements the same protcol.

Sources:

Cashu Docs: https://docs.cashu.space/

Chaumian Blinding overview: https://www.geeksforgeeks.org/chaumian-blinding/

Chaumian Blinding Paper: https://www.hit.bme.hu/~buttyan/courses/BMEVIHIM219/2009/Chaum.BlindSigForPayment.1982.PDF