If you don't know the purpose, you're not ready for them.
Differential lockers lock your two axleshafts/wheels together so they have direct 50/50 power.. that way, when you get a wheel off the ground, power is no longer 100% diverted to that tire.. the one that has traction can push you through.
Watch this video to get a better understanding:
YouTube - ARB Airlocker Operation Demo
With your Disco2, you also have Electronic Traction Control which acts somewhat like a locker, it puts some power the wheel with the traction.
