The Forcesaber is an ancient artifact of extreme power. It's origins are unknown. At some point shorlty before the foundation of the Mandalorian Empire Savage Opress obtained the Forcesaber. He used it to help the rebels fight the Mandalorian Empire. But eventually it fell into the hands of Darth Reval.
The Forcesaber will strengthen it's wielders connection with the Force. It may allow people with no Force sensitivity to use basic Force abilities. But for those that are Force sensitive it can increase their power enough to grant them all kind of new abilities.
The Forcesaber does not just give power while being held. But physical contact with the saber will also slowly increase someone's Force sensivity, permanently.