You can think of it as cash in the form of a text snippet that you can send to someone (over a secure channel).

It can be redeemed for lightning bitcoin at an authority (mint).

Anyone can anonymously redeem the token which makes it digital cash.

Why is it needed? It is much more lightweight than a lightning transaction which can involve multiple channels to transfer the money. This makes ecash better suited for a high volume of users and (small) transactions.

Right now it is very experimental but it might become an alternative way to zap when it’s better integrated.