Q. What is an embargo?

Much like how Netflix no longer produces DVDs of its shows in order to drive subscriptions to its platform, periodicals sometimes do a version of this as well to drive subscriptions to its print or higher (read: more expensive) levels of access. When periodicals do it, however, it is called an embargo.

Wikipedia has a really good breakdown of what an embargo means in the publishing setting. 

Sometimes, no library has access to even be allowed to supply via interlibrary loan due to subscription options for institutions.