It is definitely not vanity publishing or self-publishing, nor about the literature that scholars might normally expect to be paid for, such as books for which they hope to earn royalty payments. It concerns the outputs that scholars normally give away free to be published – peer-reviewed journal articles, conference papers and datasets of various kinds.
Reference: Swan, A. & Chan, L. (2009) Open Access Scholarly Information Sourcebook. Retrieved from http://www.openoasis.org/index.php?option=com_content&view=article&id=130&Itemid=390
Much more detail can be found here: