This function is a convenience function to allow you to reorder models that support the GtkDragSourceIface and the GtkDragDestIface. Both GtkTreeStore and GtkListStore support these. If reorderable is true, then the user can reorder the model by dragging and dropping rows. The developer can listen to these changes by connecting to the model's "row-inserted" and "row-deleted" signals.
This function does not give you any degree of control over the order - any reordering is allowed. If more control is needed, you should probably handle drag and drop manually.
See also: get_reorderable()