The properties are resources used as predicate of triples; the semantics of a triple clearly depends on the property used as predicate. Two things are very important with the concept of property:
First, RDF considers properties as first class object, unlike most object modeling languages, where properties are attributes of a class. Even though the concept of class exists in RDF (see subsection 2.2), properties can be defined and used independently of classes.
Secondly, as it has been mentioned in subsection 1.4, the fact that properties are resources allows to describe them with RDF itself. This will be widely used by the following concepts.