A virtual function mean that it can be overriden in a derived / child class.
In this case NotifyActorBeginOverlap and NotifyActorEndOverlap is defined in AActor. So in order to change the logic of those functions in your child class you can override it providing it’s marked virtual.
Super::NotifyActorBeginOverlap means that we are executing all functionality that was defined in the parent AActor implementation. This is to prevent ‘loosing’ anything that happens in the parent AActor implementation.
And finally yes, if anything overlaps any component in your platform actor that is set to overlap, those functions will be called.