getCurrent() is used to bind the ui.Dialog object with the currently active dialog. The method returns the current dialog object. If no dialog is active at the moment of the method invocation, it will return NULL. It is useful if you use a custom dialog object and not the default one:
DEFINE dial ui.Dialog
LET dial = ui.Dialog.GetCurrent()
There is some specifics about using variables with ui.Dialog methods.
You can learn about them here.