Open Access (OA) refers to methods of making research and scholarly publications freely available, rather than being behind pay-walls.
It is premised on the idea that publicly funded research should be publicly available.
OA publications are peer reviewed. Articles may be OA by the publisher or by an alternate source being made available post publication (either by author or institution).
Authors should balance choosing a journal by relevance and impact with consideration of equity of access.
In most subscription journals copyright is transferred to the publisher on agreeing to publication. The rights that authors retain to post or distribute personal copies of their work varies.