I read the README and the blog post, and found neither contains a “wat is this”. The repo links to an Umbral paper PDF that’s hard to read on mobile - requiring a separate PDF viewer app with tiny font - which also doesn’t contain a concrete example in the abstract.
¯\_(ツ)_/¯ Maybe I’m not the target audience.
Umbral is a split-key version of PRE, which splits the re-encryption key into many shares. A threshold number of these shares are then required to re-encrypt the data (similar to Shamir’s Secret Sharing).
Here's a post describing split-key threshold proxy re-encryption in slightly more detail: https://blog.nucypher.com/unveiling-umbral-3d9d4423cd71
We (NuCypher) are using this to build a blockchain privacy layer in the form of a decentralized key management system similar to AWS KMS or Google Cloud KMS.
You can read our paper that describes our use-case here: https://www.nucypher.com/whitepapers/english.pdf