A locker is basically a slang/abreviated term for a differential locker and sometimes referred to as a pin locker depending on design characteristics. The lockers function is to lock both drive wheels of an axle together. Without the drive wheels locked together the differential may not be directing power/torque to the wheel you need it to, in fact it may not be able to drive you at all.  A good video explanation can be found on our support page https://halo-performance.com/support