"In short, Open Access is the free and unrestricted online access to outputs of scholarly research." Open Access and Digital Scholarship Blog, Imperial College London
OA journals are available online for everyone to read for free, rather than only paid subscribers.
Many OA journals employ scholarly peer-review and rigorous editorial processes, just like commercial journals. Therefore the quality of the research product is not diminished, but access to that product is more open to students and researchers around the world.
Resources to learn more:
Your institution and/or the funding agency may require open access publishing (especially government funders). Read the terms of your funding and plan in advance. The tools below may help you to identify mandates from your institution and/or funding agency.
APC = Article Processing Charge or Author Processing Charge
Usage of some terms like "Gold" may vary, so always read a journal's policies carefully, or consult the Directory of Open Access Journals for clear information about a journal's OA publishing and author fees.
This terminology was chosen by those advocating for open access to research, so terms such as Gold and Platinum suggest a bias in favor of open. This does not make traditional journals "bad" or make all open journals "better." They are simply different ends of an access spectrum, and an individual researcher's priorities and values will determine the appropriate choice for each individual publication.