What is callback hell and how and why does RX solve it

Question

Can someone provide a precise description and a straightforward example that clarifies what a "callback hell" is for someone who is unfamiliar with JavaScript and node.js?

When (and under what conditions) does the "callback hell issue" manifest itself?

How does it happen?

Do asynchronous calculations and "callback hell" always go hand in hand?

Additionally, can a single-threaded programme experience "callback hell"?

Erik Meijer stated in one of his lectures for the Coursera Reactive course that RX eliminates the issue of "callback hell."

On the Coursera forum, I questioned what a "callback hell" was, but I didn't get a clear response.

Could you demonstrate how RX resolves the "callback hell problem" on that basic example after defining "callback hell" with a simple

example?