You can check this thread: Getting the name from a selected dropdown value
The Value of the DropDown is the property specified by ValueProperty (or a collection of those properties if multiple selection is enabled). If you don't set ValueProperty the Value will be the data item itself.
In this example ValueProperty is set to CustomerID and multiple selection is enabled. Thus the value of the DropDown is a collection of the selected CustomerID values e.g. [1,2,3]. If you don't set ValueProperty the Value of the DropDown (and the argument of the Change event) will be a collection of Customer entities.