From Xojo Documentation
A property is a value of a class or module. You can think of it as a variable that belongs to a class or module. Most of the built-in classes have properties. For example, the Date class has a property to get the Year.
You do not normally use the Property keyword in your own Xojo code. You add a property by choose the Add button in the Command Bar and choosing Property, Computed Property, Shared Property, or Shared Computed Property.
The Property keyword can be used when writing XojoScript.