Lockers is kind of a layman's term or a general term used by the automobile industry.
Lockers are specifically one type of traction device that is highly popular amongst the off-roaders.
To become an off-road hero, you have to have some sort of traction device installed in the axle of the jeep.
Without a locker installed, the jeep gets traction with only one tire. So, when you hit the gas, only one wheel gets the traction.
But once you install a traction device or locker and hit the gas, the locker locks the axle and transfers the full torque to both the wheels. This happens only for straight-line driving.