Generally anything that is not time sensitive, either because it is about a generic or static topic, or because the author makes regular updates if it is not static. EG: a post on WWII airplanes would be evergreen, as the data is pretty much not going to change at this point. Similarly a regularly updated post on stealth airplane technology could also be evergreen because it evolves with the technology and is presumably always current and relevant.

A post on raising VC funding that was written in 2007 and not updated is not really evergreeen because while it was accurate for the time, it is probably not accurate and relevant in the current funding market.

Rather than a blog post that is relevant for when it's posted, evergreen is a piece of content that stays relevant regardless of date.

A news item versus how to start a fire. How to start a fire is relevant regardless of date.

"How to fix a flat tire" as opposed to "Here's what Trump tweeted today"