Open access information is freely available online and can be used without concern of licensing restrictions. For more information, read Peter Suber's A Very Brief Introduction to Open Access.
There are many types of open-accessing publishing, but these three are particularly common: green, gold, platinum/diamond, 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. Typically gold OA publishing requires the author of work or a funder to pay an Article Processing Charge (APC).
Platinum/Diamond open access works are originally published open access but do not require payment from authors or readers. These journals are often sponsored by an institution or organization who bears any operational costs.
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.