The open access (OA) movement focuses on the removal of barriers that stand between a user and scholarly contents. As Peter Suber notes, OA literature, data, and education resources are "digital, online, free of charge, and free of most copyright and licensing restrictions."
A number of factors contribute to the rise of the OA movement. The internet has introduced the possibility of global dissemination, reach, and impact of research. It is an alternative to the subscription-based model of scholarly publishing, in which access to scholarship requires an affiliation with an institution that subscribes to journals and/or databases. Finally, both public and private funders around the world have established policies that research data and publications associated with funded research be openly shared through online repositories. (Credit to original guide writer Nora Mattern.)
This video by PhD Comics explains open access.
Funders have laid down OA policies because OA contributes to public good and the advancement of scholarship. Studies have confirmed that OA publications will be more widely read and more frequently cited than those behind a paywall. In addition to personal benefits for the author, making research openly available is in the public interest. Individuals not affiliated with an institution with journal subscriptions will be able to access and build on the published research for innovation.
The above image is made available by Danny Kingsley and Sarah Brown under the Creative Commons Attribution license.
There are a number of common myths surrounding OA that scholars and libraries have effectively debunked. Peter Suber's "A field guide to misunderstandings about open access" is a helpful starting point for understanding the quality of OA publications, the funding model behind OA, and the relationship between copyright and OA.