The Scholarly Publishing and Academic Resources Coalition (SPARC) gives us a succinct explanation of what Open Access (OA) philosophy:
Open Access is the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment.
This means that if you are the author of a paper or study, there are options available to you to help ensure greater accessibility to your scholarly work. This does involve giving up some degree of rights controlling the distribution of your work, depending on how you choose to publish.
If you are a consumer or would-be author, you may freely download and read journal articles, studies, theses, dissertations, and various other scholarly works that are published in an open access journal or repository.