QlikView properties gives you the possiblity to let the document designer change the way your extension works without having to change the code.

Some properties, like Dimensions and Expressions/Measures are used to tell QlikView how to make the calculations.

Other properties determines how standard parts of the object, like the Caption, look like and work.

And you can also define your own properties, that changes how your extension works but affects QlikView only indirectly,through your extension code.

You refer to the properties in different places.
In your Definition.xml file:
Define your Dimensions and Measures with the Dimension and Measurement tags:
<Dimension Label="Dimension" Initial="" />
<Measurement Label="Measure" Initial="" />

Set values for properties with the Initiate tag:
<Initiate Name="Caption.Text" Value="My Extension"/>
<Initiate Name="Chart.Dimension.0.Field" Value="MyField" />

Use the Text tag to define your own properties:
<Text Label="Color:" Type="color" Initial="" Expression="#FF0000"/>
<Text Label="Border:" Type="checkbox" Initial="" Expression="1"/>
In your javascript code:
In your qvpp files:
<div class='prop-grid_span-1 prop-grid_last' propicontype='singlecolor' avq='prop_dlgbutton:.Chart.Text.0.Content:Color.qvpp'></div>
You can set properties to constant values, or to an expression. If you set them to an expression QlikView will automatically reactivate your extension when the value changes.

Dimensions Control Dimensions

Measures/Expressions Control Measures

Caption, Layout etc Use to affect how standard parts like caption etc are rendered and behave.

Text Use to define your own properties.