RDF
Also called Resource Description Framework
RDF (Resource Description Framework) is a way of describing anything, a person, a document, a fact, as a simple three-part statement: subject, predicate, object. "Alice knows Bob" becomes a triple; so does "this photo was taken on this date" or "this Pod belongs to this WebID." Link enough triples together and you get a graph of facts that different systems, built by different people, can query and combine without agreeing on a shared database schema first, which is the actual point: RDF is the data model underneath Tim Berners-Lee's original Semantic Web vision, and it's what Solid uses to represent everything stored in a Pod.
RDF is a W3C Recommendation, first specified in 1999 and revised as RDF 1.1 in 2014.
Where Solidus stands
Solidus's own Pod backend stores data in Postgres, in conventional relational/JSON form, not as RDF triples. That's a real gap relative to Solid conformance, and we won't describe it otherwise. There is one honest adjacency worth knowing: Solidus's Verifiable Credentials are serialized using JSON-LD, and JSON-LD is itself a way of writing RDF, so RDF concepts do reach into Solidus's credential layer, just not into the Pod's storage layer.
Proof
Read the spec at w3.org/TR/rdf11-concepts/. To see RDF for yourself, take any RDF or Turtle snippet and paste it into a public RDF validator, it either parses or it doesn't. We don't have a Solidus-run RDF storage proof to offer, because our storage layer isn't RDF.
Where it comes from
Someone else specified this. Solidus assembles it.
RDF is a W3C Recommendation, first specified in 1999 and revised as RDF 1.1 in 2014, the data model underneath Tim Berners-Lee's original Semantic Web vision. Solid uses RDF to represent everything stored in a Pod. Solidus did not design RDF and does not use it as its storage model.
How to check this
Solidus has not built this. The entry explains the concept.
Read the spec at w3.org/TR/rdf11-concepts/. To see RDF for yourself, take any RDF or Turtle snippet and paste it into a public RDF validator, it either parses or it doesn't. We don't have a Solidus-run RDF storage proof to offer, because our storage layer isn't RDF.