The Solar Mirror Weapon was an invention by Dr. Tungren and stolen by Moloch. Apparently it absorbed light rays which it then concentrated so they could be used at will. Moloch ignored Tungren's warnings and turned it on, burning out the Professor's hand.
It enabled him and his gang to perform more difficult heists and was carried with a truck. It was used to stop the course of a military train (it apparently created an energy barrier in front of the train). But then the Minutemen showed up and foiled his heist. They apprehended the device and kept hold of it in their base.