In short, fake events.
When a library does something and the addon using the library wants to know when that finished or had results, it can assign a callback function using CallbackHandler (if the library has support for it).
For example, the library
LibResInfo will fire callbacks whenever it thinks that a resurrect is in progress or has finished in some way, and addons can use CallbackHandler to assign a function to respond to such an event.
More information:
https://en.wikipedia.org/wiki/Callba...programming%29