PFS pages occur every 8088 pages in every data file and store a byte of information about itself and the following 8087 pages. The most important piece of information it stores is whether a page is allocated (in use) or not. You can read more about PFS pages and the other per-database allocation bitmaps in this blog post.
So why can’t they be repaired by DBCC CHECKDB, when all the other per-database allocation bitmaps can?
The answer is that the is-this-page-allocated-or-not information is not duplicated anywhere else in the database, and it’s impossible to reconstruct it in all cases.
In case you’re not particularly familiar with PFS pages, Paul has a blog post from 2006 describing GAM, SGAM, and PFS pages.