While the concept is evolving, in general, it means that works such as peer-reviewed journal articles, graduate theses, data, or other types of scholarly information are made available free of charge, immediately (no publisher delay or embargo), and in a permanently accessible, online format.
For more information on open access, see these pdf files:
Lasting Impact: Sustainability of Disciplinary Repositories
PDF- an 18-page OCLC report by Ricky Erway (2012).
The following sources provide information about publisher policies with regard to self-archiving of scholarly articles.
These sources are not comprehensive; in some cases it will be necessary to contact the journal publisher directly to ask about the policies that relate to a specific journal title.