One issue in a business is what does all the data actually mean? What are the various definitions and how do they relate to each other. What's a customer? Should that include people that have bought things, or anyone who is potentially buying? Do different systems use the same definitions, what are the columns, etc.

An ontology in this kind of context means a more precise machine readable spec of this and how it all relates to each other. You can totally build one by hand. The idea here is I think that it tries to figure it out itself and can take corrections over time, so you can ask natural language questions and get more reliable results rather than hoping an llm works out what "customers this month" means the same way each time over a wide span of data rather than having a much clearer API to work against.