"Open Access (OA) literature is digital, online, free of charge, and free of most copyright and licensing restrictions" 
The basic idea of Open Access is simple: Make research literature available online, without price barriers and without most permissions barriers (aside from attribution).
Access to a single academic journal may cost thousands of dollars a year. Removing price barriers means that readers are not limited by their own ability to pay or by the budgets of institutions where they may have library privileges.
This means research is made available to everyone: policy makers, health care workers, professionals, educators, scholars in the developing world, and the public.
Although scholarly researchers have traditionally given their work away for free in order to disseminate it as widely as possible, the norm today is for scholars to transfer their copyright to intermediaries – publishers – who want to sell their work. By giving away copyright, authors often forfeit the rights to copy, distribute, or repurpose their own work, as well as the right to give others permission to do the same. By removing permissions barriers, scholars become free to access or reuse literature for scholarly purposes.
An important limit to reproduction and distribution, and the only purpose for copyright in OA literature, is to give authors control over the integrity of their work, and the right to be properly acknowledged and cited .
Open Access makes scholarly research more useful in both ways: by making it available to more people who can put it to use, and freeing those people to use and reuse it.