Open access (OA) information is free to read online (no paywalls) and free of most copyright and licensing restrictions, meaning that users can freely share and reuse the content. Check out Peter Suber's A Very Brief Introduction to Open Access for a succinct overview or Open Access Explained! for a video explaining how OA differs from traditional academic publishing.
There are many types of OA publishing, but these three are particularly common: green, gold, platinum/diamond, and hybrid.
Looking for another discipline? Check out this list of disciplinary repositories from the Open Access Directory.