(covers information from several alternate timelines)
The Klingon raider was a small starship used by the Klingons in the mid-23rd century. It was often carried aboard larger Klingon ships as an auxiliary craft. The small ships were equipped with an antimatter power supply and were capable of achieving warp speeds.
In 2256, L'Rell stole a raider from the Sarcophagus in order to rescue Voq from the abandoned USS Shenzhou and ferry him to the Matriarchs of the House of Mo'Kai. (DIS: "The Butcher's Knife Cares Not for the Lamb's Cry")
Three weeks later, Captain Gabriel Lorca and Lieutenant Ash Tyler escaped from a Klingon prison ship in a stolen raider. They were pursued by five other raiders, but were rescued by the USS Discovery before their ship was destroyed. (DIS: "Choose Your Pain")
In the mirror universe, the Klingon raider was used by rebels fighting the Terran Empire. These raiders were modified with non-Klingon technologies. The USS Discovery salvaged a data core from a raider that had been piloted by a Vulcan and an Andorian. (DIS: "Despite Yourself")