Short for manual page, the traditional form of online help available on POSIX systems. They are either maintained by the authors of the software, or the LinuxDocumentationProject, for FileFormats and so forth. The ManPages as a whole are famed (and sometimes feared) for their brevity and conciseness. New users often complain that it's hard to digest the information presented and draw the right conclusions due to its brevity, but that same brevity is frequently cherished by more experienced users.
A number of manual pages have been imported into the wiki so they can be edited. Be aware that these are not automagically updated with new versions.