Bookmark for a business-y word I now seem to see everywhere. Isn’t it odd how you tune in to something and end up noticing it more often?
In this case, “evergreen” means “timeless.” Something that ages well, keeps its value, lives a long life.
For example, The Pragmatic Programmer by Andy Hunt and David Thomas is an evergreen book on software craftsmanship. (And presumably doesn’t lose its pages in cold weather.)