More usefully, if you think of pages as corresponding to real-world objects, properties can define the characteristics and relationships of those objects. For example, if a page corresponds to a "Person" object, he might have a "Name" property, a "Description" property, and a list of "Item" properties that correspond to the things in his possession. Property values can be the names of other pages, making it easy to link pages together in structured ways.
What makes properties powerful in ProWiki is that they are tracked across the entire database, so you can pretty efficiently do lookups from one object to another. Moreover, you can construct simple queries, so that, for example, a page can automatically and easily show all of the "Items" in the database.
Next: Defining Properties