nostr:npub1cj6ndx5akfazux7f0vjl4fyx9k0ulf682p437fe03a9ndwqjm0tqj886t6 has some really well written articles. But TLDR ecash guarantees unlinkability of the transactions, meaning the Mint that gives you ecash wont be able to tell who you give that to.

The extent of this unlinkability depends on some factors, for example if the Mint has only a few users thats not very private or through amount fingerprinting. Lastly -and this is lesser known- if your cashu wallet is checking the state of sent ecash (in cashu.me you can disable it) that also could break the unlinkabilty.