Kudos to the Princeton faculty for voting this in. (Duke made a similar move a little over a year ago.) In a nutshell, all faculty publications (research papers, books, etc.) will be made available to the public, at no charge, by the University. This is a major face-slap to publishers like Elsevier who require faculty to sign over their copyrights and subsequently don't allow faculty to post their work on the web.
Princeton and Duke are top-tier institutions; their leadership will be important for encouraging other institutions' faculties to adopt similar policies.