Open access (OA) resources are digital, online, and free to use. They also often carries less restrictive copyright and licensing barriers than traditionally published works, offering both users and creators a level of access to their work otherwise impossible under strict paywall models, and promoting the spread of knowledge unimpeded by the current structure of scholarly publishing. An easy way to remember what OA is: "free availability and unrestricted use."
And just because they're free, that doesn't mean OA materials are not legitimate! While open access is a newer form of scholarly publishing, many OA journals do comply with well-established peer-review processes and maintain high publishing standards.
If you want to get a sense of the leaders in the OA community and how they show that open access can thrive within the scholarly community, check out Hathi Trust and the Directory of Open Access Journals.
In regards to publishing, open access (OA) stems from your right as a copyright holder to make your work available digitally, online, free of charge, and less restricted to copyright and licensing requirements than in traditional publishing methods. Open access can enable both users and creators a level of access to their work otherwise impossible under strictly toll access models, and promote the spread of knowledge unimpeded by the current structure of scholarly publishing, chilled by paywalls and embargos.
Because authorial consent fuels open access, a number of versions, each of which confer different rights, exist to fit the specific wishes of the copyright holder. This means that participation in the open access movement does not imply that you will necessarily lose the ability to publish in traditional journals, and reap benefits from doing so.