meaning the machine can use
An ontology is the layer where data becomes meaning a machine can use. Most systems store records; an ontology stores what those records mean and how they relate, so software and people reason over one shared model. It is expressed in open standards, it resolves identity so the same real-world thing becomes a single object, and it does not only describe the business, it can act on it. Read it from the sources at the bottom up to what consumes it at the top.