A property is kind of like a variable, which can be used when you want values that drive configurations of visualizations and calculations to be easy to modify by others.
To exemplify, assume you are trading with foreign countries, and the figures you want to analyze are dependent on the current exchange rate. Then you can create a property that represents the exchange rate, and use the property instead of a fixed exchange rate in all conversion expressions. As soon as the exchange rate changes, you adjust the value of the property to the current rate, and recalculations are performed at all places where the property is involved.
Properties can be used at many places, not only in calculations. Other use cases could be, for example, to specify what to display on visualization axes, how to color items, or how to define lines in visualizations.
Extremely valuable is that the property values can be modified from controls in text areas.
Note: Property controls in the text areas work also in the web client.
The process to follow when setting up property controls is:
Decide which values in the analysis the user should be able to control from the text area.
Create a property, which can assume these variations in values, and its property control.
Hook the property to the place where user control is wanted, for example, an axis selector or an expression.
The best way to explain these steps is through examples. They also give you a hint of what is possible to do.