What is Open Access?
Open Access literature is freely available, peer-reviewed, online scholarly literature which is free of most copyright and licensing restrictions.
This means it can be freely accessed by anyone in the world, with the potential readership of open access articles being far greater than that for material where the full-text is restricted to subscribers.
The legal basis for open access is usually the consent of the copyright holder.
The Plan S Principles offer a useful introduction to open access for scholarly communications. https://www.coalition-s.org/plan_s_principles/