Open access literature is information that is available online free both monetarily, and in terms of copyright and licensing restrictions. Making information publicly available instead of placing it behind a paywall dramatically increases the number of people who are able to read and cite it, and increases equity of access. 

Within the context of scholarly publishing, the open access model stands as a sustainable alternative to the traditional subscription access model

The following short video does an excellent job of explaining what open access is, as well as the benefits.

There are several benefits to publishing open access enjoyed by authors. Some examples include:

• Broader access means a broader audience, which increases the visibility and impact of your work.
• Readers have barrier-free access to your work, instead of being restricted by subscriptions and paywalls.
• Increased visibility also increases opportunities to collaborate with other experts within your field.
• Making your work open access may be required by funding agencies.

For an excellent (and continually updated) overview of how to make your work open access, please see How to make your own work open access, hosted by the Berkman Klein Center for Internet & Society at Harvard University. In particular, they provide many useful links for authors interested in open access publishing.