Copyright allows an author to place a license on their work, granting others greater freedoms, sometimes along with certain specific obligations. So it is important to understand that copyright allows an author or publisher to be very restrictive, and it also allows an author or publisher to be less restrictive. A license makes these less restrictive terms explicit, and the ability to control these terms is made possible by copyright.
Generally an open license allows unlimited copying. It often allows the creation of derivative works, and the mixing of material from a variety of openly licensed documents. A viral license obliges the author of a derivative work to grant the same license to the derivative work, rather than asserting more restrictive terms. The licenses are usually perpetual, so they do not expire at a fixed term.
It is easy to get distracted by legal jargon, obtuse arguments, and misunderstandings. We view an open license as statement of intent. The work is free to use forever. It will not go out of print. If you send the author a correction, suggestion, or contribution, it can be incorporated and enjoyed by others freely. And should an author lose interest in a project, or become unable to continue working on it, another individual may take it up and continue to maintain it.