Custom events are an often overlooked feature of VBA. I would say well over 95% of all of the projects I have worked on do not use any Custom Events, yet they are a very helpful tool which have a variety of applications.
I have put together two videos that help illustrate how Custom events work, and an example of how events can help you in your application.
Custom Events: https://www.youtube.com/watch?v=kfIabBuUtmA
Custom Events Example: https://www.youtube.com/watch?v=tVbUwdfm57Q