Basically, lockers mechanically lock the right and left wheels together so one side can't spin ineffectively and leave the other tire doing nothing, leaving you stuck. Some lockers do a "hard" lock so each side is locked to the other side no matter what. Their purpose is to increase the traction available to you to help keep you from getting stuck by forcing both tires to rotate together. ARB Air Lockers, Ox Lockers, etc. are like this when energized via a push-button. They are either completely locked or they are totally unlocked like an open axle.
Other lockers that are classified as "automatic" like the Detroit Locker, Lockrite, Aussie, etc. allow one side to rotate faster (it temporarily unlocks) than the other like is needed when you are making a turn... then it re-locks once the turn is complete and you are driving straight again.
Do read one or both of those FAQs at the top of that page... sorry they're a bit long.
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