Content decay is when a page that used to bring steady traffic slowly loses rankings and clicks, usually because it's out of date or competitors published something better.
Why it matters
It happens quietly. A post that brought 900 visits a month can slide to page two over a year, and nobody notices until the traffic is gone.
Example
A "best coffee grinders of 2024" article keeps ranking for a while, then drops as searchers and Google prefer newer, updated lists.
What to do
Watch your pages' clicks and positions in Search Console, and refresh slipping pages: update facts and dates, add what newer competitors cover, and improve the answer. Keep the same URL.