Properties are defined using a homebrew syntax, inspired by UseMod?'s FreeLinks? syntax. It's completely made-up and simplistic, but easy to use.
To define the "IndexName" of this page, you would say:
{{IndexName=Defining New Properties}}
When you edit the page, that gets interpreted and stored in the database of the "IndexName" property. When you display the page, it comes out colorized, like this:
IndexName = Defining New Properties
Note that "IndexName" comes out as a hyperlink. If you click on that, it pops up an index of all pages that contain the Name property, and their values. Feel free to click it and come back -- I'll wait.
Each page may have any number of properties, and they can be pretty much anything. In theory, you can use any name that works as a FreeLink? for a property name, but this is not heavily tested. I recommend sticking to alphanumerics and maybe spaces for the time being.
Pages may have multiple values for the same property. For instance, a "Person" page may have a dozen "Item" properties. This is entirely valid -- ProWiki intentionally allows multi-valued properties. In fact, all properties are treated as multi-valued internally, although in many circumstances it expects to find only one. (Nothing will break if it finds multiples in these cases, but it will choose one at random.)
Here is another property definition; you'll see in a minute why I'm including it:
IndexAbstract = This page describes how you can define arbitrary properties on a page.
Next: Property Queries