"Open Access" is unrestricted access via the internet to peer reviewed scholarly research. Open-access (OA) literature is digital, online, free of charge, made available through the internet, including for those who do not have access to personal or institutional subscriptions to journals, and free of most copyright and licensing restrictions.
An advantage of OA over traditional publishing models is a higher impact and visibility of research, since that research reaches a wider audience. Research that is open access can therefore result in increased collaboration opportunities and citation impact.
There are two videos that explain this further (opens YouTube):