A seed is a plan or outline for a page. It's a quickfire list of things that the final page will include (often in bulletpoint form), and won't be much help for readers trying to learn about the page's topic. Seeds are useful for authors wanting feedback before writing up full pages,and for writers who want to quickly sketch out lots of pages to lay the foundations of a project.
When converting a seed into a full page, it's often good to change the unfinished parts into TODOs.