Evergreen content is content where the core concept remains the same, but the examples and content change each time, so the student will have a new experience each time they take the class.
An example would be something like "Easy Rock Songs for Guitar.” As a prerequisite, some basic knowledge of open chords, barre chords, and strumming would be critical.
While each class will be almost the same in core concepts (putting 4 chords together, understanding song form, etc.), subsequent classes can use different song content and examples each time.
So the same student can sign up for this class ad infinitum, and have a wonderful experience each time building new repertoire.
Other class topic examples may include:
Simple Songs for Piano Beginners
Easy Rock Guitar Solos
Basic Blues Improvisation
Learning Tunes: More 4 Chord Songs
GarageBand for Songwriters