Well, put simply, an "accessible copy" is a version of a work that provides for improved or easier access for a visually impaired person. This means that possibly even the original format of the work may need to be altered. An accessible copy can include added capabilities for navigating around the original version, so it includes both hard and soft copies. In other words, if a person is totally blind, a text can be made into an audio format so the person can listen to the book, or it could be translated into braille. What if a person can see, but cannot hold a book? This person is also included in the context of visually impaired and can have books made into e-books and the like for easier use which also comes under the definition of an accessible copy.
With audio books being published and many libraries now lending them, certain types of accessible copies are much easier to come by these days.