UNESCO define open access (OA) as "free access to information and unrestricted use of electronic resources for everyone"
Traditionally, scholarly activity, in the form of research articles, has been published in journals by commercial academic publishers who charge for access to the research. This is why some of the resources available through the library are password protected; they are provided by commercial academic publishers who licence their content and expect to be paid for making their content available.
Many people think it's unfair to limit access to research, a lot of which is funded by governments, with the money for this being raised by taxes. So you pay income tax to the government, some of which goes to fund research, which gets published in a journal that comes from a commercial academic publisher and you would have to pay to read the article. It doesn't help that commercial academic publishing is very profitable
Recent years have seen the growth of an Open Access movement to make research outputs, like journal articles, freely and widely available online and many governments now have a rule that all publicly funded research must made freely available.
So basically, if a scholarly publication is Open Access, it's freely available.
It's not just scholarly articles and journals that can be Open Access, textbooks and other scholarly books can be Open Access too
Further Reading
Digital Library Open Access section
UNESCO: What is Open Access?
The Budapest Open Access Declaration
