Open Access refers to the practice of making research works, such as peer-reviewed articles and other data, freely available on the web, to help promote scholarship and research around the world.
Starting in the early 2000's, many universities established Institutional Repositories, or web sites where they could preserve the scholarly output of their institutions and make it available to others. Dissertations and theses were obvious candidates for inclusion, along with faculty authored works and other kinds of university publications. For an example, see:
Open Access to scholarly research got a major boost in 2008 when the National Institutes of Health mandated that peer-reviewed articles based upon NIH-funded research must be made freely available. One of the most visible results of this mandate was PubMed Central. Many countries now insist that publicly funded research be made publicly available.
Other open access repositories provide research in progress or research data that may not have made it into print. A major example is arXiv, for physics, mathematics, and other science disciplines.