An RDF repository is the data base where RDF data is stored - essentially a DBMS. The repository also provides access to the stored data as an RDF Graph, which can be retrieved like a file, or via a Linked Data Query. Most RDF repositories provide both kinds of access.
Since the atomic unit of data in RDF is a Triple, RDF repositories are also called "triple stores". However, most triple stores also store other information with each triple so this name can be confusing.
RDF data may also be dynamically created from other formats, such as from an SQL or XML DBMS. Such a "virtual RDF Repository" can be indistinguishable from a repository that stores RDF data natively. There is software and standard for exposing existing data as RDF, some dynamically and some by replicating the data in a triple store.