This is the action of providing unrestricted access to material that would normally be subject to copyright restrictions. Some material may be initially restricted then at a later date become available via open access, some material may be published as open access in the first instance.
"In the Open Access environment, authors generally publish under the Creative Commons Attribution License which allows for free and immediate access to, and unrestricted re-use of, original works of all types. Under this license, authors make their scholarly content legally available for re-use, without permission or fees, and for virtually any purpose. Anyone may copy, distribute, or re-use these articles, with the condition that the author(s) and original source are properly cited." (Yusuf Ras, Stellenbosch University).
Some publishers who are accepting open access material: