RDF is expressed using subject-predicate-object triples, where both the subject and predicate have to be URIs, but the object may also be some form of literal value such as a number or string. This allows one to express statements such as:
<https://alandix.com/academic/> <ex:is_about> "Alan Dix".
The abstract triples may be represnted in a number of concrete syntaxes, some more compact, some easier for human or computer processing.
Used in Chap. 17: page 252