It means the don’t pay enough to attract and retain qualified candidates via normal means.
Companies pay to list their job higher in search results and other ad spots. Usually means they're farming for resumes, don't have a good idea of what profile they're looking for, or simply aren't getting that much interest (there's probably a reason for that too).
Not much interest ? Why would it happen ? Low salary ?
Companies probably need someone now/soon. Not just an openjob to pool candidates. So they paid for those to be shown as promoted