DITA maps are documents that collect references to DITA topics to organize them for output to a specific deliverable, including generating navigation files and links to related topics.
A typical DITA map has a title and a set of topicrefs, in a sequence, in a hierarchy, or both, that define the structure by which DITA topics are to be viewed as a complete information deliverable.
Figure 1. DITA Map opened in Serna.
You can find more information about maps at: http://docs.oasis-open.org/dita/v1.1/OS/archspec/maps.html.