Open Access (OA) refers to the free access to and reuse of scholarly works. Academic research has traditionally been published in peer-reviewed journals, monographs and collected editions. These publications have increasingly (if not mainly) been accessed as online publications through academic libraries, which pay often-hefty subscription fees to make this content available to its students and staff. Different universities and their libraries in the UK and around the world have wildly-diverging budgets, and this means that not all institutions can equally afford to pay for the same type or amount of journals.
There are currently two main forms of delivering OA: the 'Gold' model, in which the peer-reviewed, professionally copyedited 'version of record' is published on a journal or platform and is openly accessible (this means no paywall, but also an open license for re-use and re-distribution) and the 'Green' one, in which academic content is made available via institutional OA repositories or archives.
Most institutional repositories are not peer-reviewed, but authors are advised to use them to deposit versions of their published articles under the conditions allowed by their publishing agreements. Most of the times publishing agreements will only allow authors to deposit their published research (also called 'self-archiving') after a period of embargo, which can be lenghty, especially for a digital age. (You can find out your publisher's copyright and self-archiving policies here).
Since the financial sustainability of Gold OA publications does not depend on subscriptions paid by university libraries, many publishers have opted for Article Processing Charges (APCs) which are meant to be covered by the author, or, ideally, the author's research funder. Not all Gold OA publications rely on APCs, and not all APCs are the same. Some Gold OA journals, for example, offer fee waivers to students or those not able to cover them for any reason, or offer memberships encouraging universities to cover their research staff's APCs.
In June 2012, the National Working Group on Expanding Access to Published Research Findings (led by Dame Janet Finch CBE) published their report, titled 'Accessibility, sustainability, excellence: how to expand access to research publications' [PDF], where the Gold model is given preference. In February 2013 the Higher Education Funding Council for England (HEFCE) made a call for advice on developing the four UK funding bodies’ joint policy on open access in the post-2014 REF, and on 24 July 2013 they followed it up by launching an online consultation inviting “responses from higher education institutions and other groups, organisations and individuals with an interest in scholarly publishing and research”. (Read the consultation here).
Yesterday's morning the UK Parliament's Business, Innovation and Skills (BIS) Committee published a report including their conclusions recommendations on the UK government’s policy on OA. You can read the conclusions and recommendations (it won't take you long) here: http://www.publications.