"Open Access" generally refers to scholarship that is free to read and re-use.
SPARC's definition of Open Access.
Peter Suber's definition of Open Access.
Wikipedia's entry for Open Access.
When it comes to making your work OA, there are two different ways to do this:
Green OA: Publish in a subscription journal but keep your rights and archive a copy of your article in an disciplinary repository. This allows authors to meet the public access requirements that many research funding agencies have.
Gold OA: Publish in an Open Access journal.
There are two other distinctions to keep in mind when talking about OA:
Libre OA: Scholarship that is free of most licensing and copyright restrictions.
Gratis OA: Scholarship that is free to read, but still has licensing and copyright restrictions.