Evergreen content is stuff that stays useful long after you post it. That's different from news, trends, or a sale that's over in a week. "How to choose the right HVAC system" is evergreen. "Our Black Friday sale" is not.
Why Google likes it
A really useful evergreen page tends to climb slowly and keep climbing, because people keep finding it, linking to it, and reading it. One strong post can bring in free traffic for years. A social post is gone from the feed in a few hours.
That's also why I'd lean on it now more than ever. Google is answering a lot of quick questions on its own, which is part of why some sites saw traffic drop. Posts that answer real questions in your own words hold up better than generic ones.
It builds trust before they call
When someone is trying to figure out a problem, they go with whoever explains it clearly. If your page is the one that helped them, they already trust you by the time they pick up the phone.
It feeds everything else
A good evergreen post becomes the raw material for the rest of your marketing. Link it in an email, pull a tip for a social post, or send it to a customer who asks the same question. Here's how one post turns into several pieces, and how to stay visible without posting every day.
Building a small library of these pages is usually a better use of time than a constant stream of posts that expire. It's a big part of what SEO and content work is really about.