In its most basic definition, metadata is just “data about data.”
The concept of metadata is not new. In fact, librarians have been using it for a long time to discover, and manage documents. Imagine, that for each document you’re specifying the author, date, book length and so on. That is all metadata that helps classify a book. Therefore, it makes it easier to find it later on.
To work appropriately, metadata has to follow a logic of classification that everyone understands. In short, there must be a set of rules that everyone can follow to make the system work. Like in a vocabulary where grammatical rules are arbitrarily selected to create a standard language. Ontologies are the foundation of metadata.
The simplest form of an ontology is a vocabulary. The vocabulary that today makes Semantic SEO possible is called Schema.org.