Open access research is research that doesn't exist behind pay walls. Pay walls are anything that requires you to pay money to access the content. Many scholarly journal articles require someone to pay money for access to that research content. Usually it's the college or university, through the library, who pays for access to the articles. A library can't afford to pay for access to everything that is being published, so some research articles have to be requested through interlibrary loan or paid for by individuals.
Open access books and articles are different. Anyone can access them for free. However, it's important to evaluate their quality before you use them in your research (you should do this with all sources of research and information, regardless of how you obtain it).
Qualities to consider