There are many types of open-accessing publishing, but these three are particularly common: green, gold, and hybrid.
Green open access works are versions of previously published works that have been self-archived in an open-access repository, such as ISU ReD or arXiv.org. Depending on rights retained by the author and the publisher's policies, work versions could be pre-prints (prior to peer review), post-prints (after peer review but before publisher layout and typesetting), or the publisher PDF/version of record (final version that appears in publication). Check the publisher's policies for each work before self-archiving, or ask a librarian for help.
Gold open access works, such as journal articles, books, and book chapters, are originally published open access. Sometimes gold OA publishing requires the author of work or a funder to pay an Article Processing Charge (APC).
Hybrid open access works are single articles that are published in a subscription journal but are made open access through a fee. The author or a funder pays the Article Processing Charge (APC). You may be able to pay an APC through grant funding, or contact your department to see if funds are available.