"Anonymous" of course means "without a name" and is used when the author is not known--or sometimes, when a story develops out of an oral tradition over generations with possibly many storytellers contributing to and revising the tale before it is finally written down and becomes literature.
A notable amount of ancient and medieval literature is anonymous. This is not only due to the lack of documents from a period, but also due to an interpretation of the author's role that differs considerably from the romantic interpretation of the term in use today. Ancient and Medieval authors were often overawed by the classical writers and the Church Fathers and tended to re-tell and embellish stories they had heard or read rather than invent new stories. And even when they did, they often claimed to be handing down something from an auctor instead. From this point of view, the names of the individual authors seemed much less important, and therefore many important works were never attributed to any specific person.