A property is a piece of configuration you can set, which guides the process of normalising your data.
Properties help your Bunker understand your dataset's original schema.
For example, you might have several original columns - such as Street, Town and Country - which together are categorised as an Address. In most countries, addresses include a postal code (or zip code), which unambiguously identifies the street.
So the Address category comes with a Postal Code property, which tells your Bunker which of the original columns contains the code. When you use your Bunker to assign a group of columns to the Address category, you can also configure that property.