From what I can tell it is a library for standardising return values that is independent on the transport medium. A HTTP service may return a particular http code or body, and gRPC would have its own representation.

I think this library maps the different representations into a single representation. The business logic will use the single representation instead of tightly coupled to the transport medium

Example from the docs:

    PAYMENT_DECLINED
    |
    +-- HTTP 422
    +-- gRPC FAILED_PRECONDITION
    +-- logs/metrics keep PAYMENT_DECLINED