Light Hits scored by a weapon with the Light trait against a Vehicle, Super-heavy Vehicle, Knight or Titan model are automatically discarded, before any Save rolls are made. In other words, a weapon with this Trait cannot damage models from the listed Detachment types.
In addition, Hits scored by a weapon with this Trait cannot be allocated to Void Shields, regardless of the weapon's AP, and thus are automatically discarded if a target has active Void Shields.