Wikipedia explains it well: “The subject denotes the resource, and the predicate denotes traits or aspects of the resource and expresses a relationship between the subject and the object. For example, one way to represent the notion “The sky has the color blue” in RDF is as the triple: a subject denoting “the sky”, a predicate denoting “has the color”, and an object denoting “blue”. Therefore, RDF uses subject instead of object or entity in contrast to the typical approach of an entity–attribute–value model in object oriented design: entity sky, attribute color, and value blue. ”In the early 2000s, another syntax for describing metadata appeared: Microformats. This was introduced by the WHATWG HTML working group and proposed a much simpler view of the Semantic Web.
It is based on several existing standards but used an easier to understand format. The goal was to “write stuff once”, and add context to human readable content. Unfortunately, the Microformats syntax was unambitious and lacked expandability. Plus, by tying the human readable content together with the machine readable code things became fragile. It became harder and harder to maintain both and to fix things whenever something changed.