Hardened Resistors increase your Explosive and Projectile resistance, enabling your ships to fight effectively at long range, against both Defense and Outpost targets.
Hybrid resistors are much more mass efficient than individually equipping two basic resistors. They also have no repair time and actually increase the repair efficiency of your cruisers as the added resistance effectively grants extra armor.
Hybrid resistors lack Alien resistance; be wary of advanced Xeno/Alien weaponry!
It was originally intended to cost 4%/7%/10% ship mass.