It means that the Supergirl film will be re-released on DVD by Warner alongside the huge Superman DVD collection coming out later this year will additional footage not seen in theatres.
However, there are versions of the film that have been previously released on DVD by Anchor Bay. I believe these releases included a 2-disk limited ed. DVD and a Director's Cut. I recommend buying the Director's Cut if it is not included in the upcoming re-release.
*Edit*
The limited ed. DVD includes the Director's Cut.
Thx for the reply! I already have that 2disc edition wich is awesome. I just thought new material would be added...
So by the end of the end? thx