Repositories are pluggable RepositoryFacade implementations. These allow us to switch out the repository that performs reading/loading of a MOF model (i.e. the MDRepositoryFacade is an implementation of the AndroMDA repository for MDR).
AndroMDA by default reads models from XMI files. If you wanted to change the default behavior of AndroMDA so that it could read a model from another format, or repository other than MDR, you would implement another RepositoryFacade implementation and place it on your classpath instead of (or before) the default MDR repository jar. Please read Developing a Repository for more information on how to develop and use your own.