Access, use, modify and re-use the content online. The adjective “free” is commonly used in one of two meanings: “for zero price” or “with little or no restriction”. Users can potentially do two kinds of things: (1) access and use it and (2) modify and re-use it.
Under this kind of access the text of published research articles are accessible free online for all. All authors, without exception, give their articles away for free. None seek or get royalties or fees from their sale. Under the spirit of this access research articles have always been intended for modification and re-use. There are no copyright barriers to modifying, developing, building upon and re-using an author's ideas and findings, once they have been published, as long as the author and published source are credited.
Synonyms: without a price-barrier, without a permission barrier