@Rozelle-ly Basically, it means the developer has completely stopped working on the extension. There won't be any new updates, bug fixes, or support.
To answer your question about what probably happened: since a lot of these extensions are free, solo side-projects, developers often just get busy with life or move on to other things and don't have the time to maintain them anymore.
The extension might still work for a bit, but as the browser gets updated over time, it will eventually break permanently since no one is around to update its code.