The idea builds off many other note-taking philosophies like Niklas Luhmann's Zettelkasten method Andy also wrote a good post Comparing Evergreens vs Zettles, and Sonke Ahren's How to Take Smart Notes system.
My personal notes system has been heavily influenced by the Evergreening ethos. In both my Roam Garden and on this very Digital Gardening website. My notes taxonomy flows from "Seedlings" to "Budding" to "Evergreens"
To help myself internalise some of the main Evergreen principles, I illustrated them out below. These aren't a comprehensive explanation, so I recommend reading the original source material if this piques your interest.