Property class items are like templates for objects you can place in your forms (new objects can inherit the properties). For example, let's say you have several customized standard buttons on all your forms that use the same color and sizing scheme. You can insert a button and have it inherit a property class. That will take care of the sizing and other visual attributes for you. You can also add triggers to button classes, and those will create a template trigger code.
You build the property class by adding and setting properties. When you create a new property class, there is very little in it. Click on the add property button, and the various form properties appear in alphabetical order. Select a property, and it is added to the property palette.