What is evergreen content? It's something you create -- a blog post, a video, a training course -- that doesn't have an expiration date. Or, if it has an expiration date (everything is evolving all the time!), it will retain its value for at least a couple of years.
I never considered this concept when I started my business or when I chose my focus of Facebook advertising. Had it been important to me at the time, it's possible I would have gone a different direction.