A statement is a resource reifying a triple. Such a resource must have at least 3 properties3: rdf:subject, rdf:object and rdf:predicate, valued by the corresponding resources.
The reification of triples may seem a utility concept rather than a fundamental concept. Nevertheless it is defined as a part of the model in the W3C recommendation. This supports the will to use RDF as its own meta-system, to make every element of RDF describable in RDF itself.