Cashu is e-cash reinvented. It goes way back as pretty much being the OG cryptosystem back in the 80s made by David Chaum. @npub12rv5lskctqxxs2c8rf2zlzc7xx3qpvzs3w4etgemauy9thegr43sf485vg is the resident master of e-cash here and is a lot more familiar with it but it's essentially an off-chain way to on-ramp/off-ramp/utilize the Bitcoin system.

Basically it translates bitcoin into tokens that exist on your device that you can use in apps - almost like an an arcade where you can get your money back for the unspent bitcoin you've used.

Cashu/nostr wallet has been the only system I've found that allows the best possible UX for most users, who are largely untechnical - simply scan a QR code or use the receiver address generated with Cash App (used by ~60 million young Americans) and you can pretty much create bitcoin experiences seamlessly.