In the Information Exchange Layer we can represent knowledge in the same way we can state Facts and communicate about these Facts. Representing a Fact is pretty simple, it is somehow in RDF like in pretty simple sentences in our Natural Language, because every bit of knowledge, every fact consists out of a Subject (what it refers to), a Property (which means the Subject has a specific Property) and an Objects (which means the Property has a specific Value).
In RDF everything can in principle a resource and resources are identified via unique URI. Resources need to be described. This is done with the help of properties and Relationships among these resources. This Relationships can be represented as Graphs. RDF provides a Framework which is a combination on Web-based protocols and it is based on formal model which means that it has a formal Semantics that is well defined and is sound. This allows to identify Relationships among Resources as well as to handle this knowledge with a formal Semantics that we can deduce new knowledge from implicit hidden knowledge inside this Resource Description Framework.
Subject and Object can be resources and these resources can be addressed via URIs. The Property is nothing else than the attribute for description of resources is also an URI, i.e. it can be denoted with an URI. The property has a value and value might be another resource or might also be, if it is descriptive data, a string or simply characters (i.e. Literal).
Thus, RDF Triples are composed of three parts: subject (which is a URI),a Property (which is also an URI) and a Object which might be an URI, if it is referring to another Resource or it might be a Literal, if it referrers to a String or a Value that has an associated Datatype.
Hendler, J. and McGuinness, D.L. (2000). The DARPA Agent Markup Language. IEEE Intelligent Systems 16(6): 67-73.