The vbucket map is used to figure out which node a vbucket is located on in the cluster. It is persisted into a file on each node as part of a cluster configuration file. Unless you’re developing your own Couchbase SDK you shouldn’t need to worry too much about this since all tools and SDK’s should use the vbucket map in order to route requests for you.
As for the map, you will need to be more specific what your asking about. If you can point to a place where a map is referenced then I can help explain what it means.