A battle saber was a type of lightsaber constructed by the Jedi Order in the Jedi Temple armory. These lightsabers were designed to be simplistic, not much different from a simple cylindrical metal hilt, and to be quickly produced and in large numbers if need-be, most often in cases of galactic emergencies.
Many of these were constructed during the Nihil threat during the High Republic Era. During that time, the Jedi Order enacted the Guardian Protocols to combat the Nihil after their destruction of Starlight Beacon, and instructed Jedi Master Govena with helping others construct the sabers.
During that period, fights between the Jedi and Nihil would escalate, meaning more lightsabers would be lost, and more would need to be available to give out to able-bodied Jedi in those events. Due to the sabers' simplistic design, they were generally disliked by the Jedi of the High Republic, as their standards for their sabers were often luxurious pieces of art, as opposed to the bare-bones design of the battle sabers.
Schematics of the battle saber could be found in Volume Five of Professor Huyang's technical manual.