What is the executor pattern in a C++ context?

The author of asio, Christopher Kohlhoff, is working on a library and proposal for executors in C++. His work so far includes this repo and docs. Unfortunately, the rationale portion has yet to be written. So far, the docs give a few examples of what the library does but I don't feel like I'm missing something. Somehow this is more than a family of fancy invoker functions.

Everything I can find on Google is very Java specific and a lot of it is particular to specific frameworks so I'm having trouble figuring out what this "executor pattern" is all about.

What are executors in this context? What do they do? What are the canonical examples of when they would be helpful? What variations exist among executors? What are the alternatives to executors and how do they compare? In particular, there seems to be a lot of overlap with an event loop where the events are initial input events, execution events, and a shutdown event.

When trying to figure out new abstractions I usually find understanding the motivation key. So for executors, what are we trying to abstract and why? What are we trying to make generic? Without executors, what extra work would we have to do?