"Open Access is the free, immediate, online availability of research articles coupled with the rights to use these articles fully in the digital environment. Open Access ensures that anyone can access and use these results—to turn ideas into industries and breakthroughs into better lives." SPARC
Open Access "is a system of free, unfettered access to scholarship, learning content, and data in a way that allows it to be widely shared so that all who wish to gain access to it may do so, free of paywalls, access fees, subscriptions or other barriers." Stephen Bell
Why is this important? It allows anyone access to scientific knowledge so that they can build upon it. Because of Open Access, a high school student, Jack Andraka, was able to develop a cost-effective diagnostic test for pancreatic cancer and win the 2012 Intel International Science and Engineering Fair. This was possible because the research he used was from content freely available in PubMed Central.
There are two primary models of Open Access publishing:
Often when doing research, even in library subscribed databases, you may come across an article that you do not have access to. If you are searching on Google Scholar and that happens, you may be asked to pay to access the article. That's called hitting a paywall. To see if there's an Open Access version of the article, try this
"Open Access Explained" by Jorge Cham, Nick Shockey, Jonathan Eisen licensed under CC BY
"Open Access in less than 4 minutes" by Hornswoggle licensed under CC BY-NC-SA 4.0