Essentially this is a language agnostic framework for building data processing systems that are highly-avalible, distributed, topologically static (no dynamic scaling), and features exactly once processing.
You define a message handler that will always produce the same output sequence given the same input sequence and the framework provides delivery, serialization, buffering, durability and transparent recovery. They even provide a nice way of wrapping non deterministic behavior so that you can seamlessly continue even if you fail in the middle of processing a message.
That being said, the really are sloppy with their performance numbers, the comparison to gRPC isn't really fair at all due to their dynamic batching. And the code examples in the paper have some really silly errors.
But the paper is still a great introduction to reliable stream processing and basic strategies for delivering exactly once delivery.
Also, there is some interesting code that the paper glosses over in this repo: https://github.com/Microsoft/CRA