It's an internal project. It basically takes a "diff" from Phabricator, and goes through the process of applying it to master and ensuring that all unit and integration tests pass before committing it.

This removes the need for every developer to do a rebase, run test, attempt land, discover conflict, rebase, run test, ... loop.

It's an internal system, but it was discussed at F8 last year. Here's the video (https://www.youtube.com/watch?v=X0VH78ye4yY#t=31m30s) and someone's notes on it (http://gregoryszorc.com/blog/2015/03/28/notes-from-facebook'...).

Probably an internal sister project to Sandcastle, another Facebook project mentioned in the post.

Seems to be an internal Facebook tool.