It's about sites and pages that can be easily used by the visually impaired - they handily provide info about it in a VERY LARGE FONT, which in a nutshell, demonstrates what it's about. http://labs.google.com/accessible/faq.html
Of late, Google is learnt to have started its experiment on accessibility of websites which it could implement on a full-fledged basis in the near future. If it happens, then the websites which are in conformity with Google’s Accessibility Protocols will achieve higher ranking in its SERP. This initiative of Google is congruent with the W3C standards which aim at establishing a set of coding standards to be followed by all websites in order to bring stability and uniformity among websites coding for the benefit of webmasters and visitors. Google’s Accessibility Web Search is planning to make the websites accessible to the visually impaired web searchers. Google’s guidelines on accessibility which is still under development may not be as strict as W3C Web Accessibility Initiative; however they will bring a world of difference to the visually impaired web users.