eCash is a concept for anonymous electronic cash. It's seeing a renaissance with Bitcoin as the monetary base (instead of fiat). You interact with a custodial mint and send your BTC (onchain or via LN) to a mint. The mint, in return, gives you eCash tokens. Those tokens can be redeemed by anyone at the mint for BTC.

In the case of the payment for silent.link, I handed ~15000 Sats in eCash tokens to the mint, and the mint paid a Lightning invoice on my behalf. The mint doesn't know who I am, so the Lightning payment is completely disconnected to any identity.

Hope that helps a bit.