Polaron weapons are energy weapons used by the Jem'Hadar, whose beams initially passed straight through Federation shields.
When the Jem'Hadar made their first major show of force in 2370, their ships' phased polaron beams cut through the shields of Federation vessels as though they were not there. The Galaxy-class USS Odyssey was destroyed in that encounter, giving the Federation an early warning of the threat posed by the Dominion.
Starfleet later modified its shields to resist the weapons, and polaron fire remained a hallmark of Jem'Hadar ships throughout the Dominion War.