PostEaseWrite for a search query, not a scroll
Since Pinterest behaves closer to a search engine, a description should answer a specific question someone might type. Say what the Pin shows and what problem it solves, rather than writing a casual caption for a feed someone is passively browsing.
Front-load the most specific, relevant words
The clearest, most specific description of what the Pin is should come first, not buried after a few lines of introduction. A reader, and the system matching Pins to searches, should be able to tell what the Pin is about from the first sentence alone.
Match the description to the board it lives on
A description that reinforces the same specific topic as its board strengthens both. A Pin about "small kitchen organization" on a board named exactly that is a clearer match than the same Pin sitting on a broad "home" board with a generic description.