A Locking is a thin locknut used to keep the fixedgear sprocket from coming unscrewed. Fixedgear hubs use lockrings to keep the sprocket from unscrewing when cycling.
So a lock ring is only neaded for fixed gear cyling. The lockring is a threaded ring / washer used to prevent the sprocket from becoming loose during rotation on the track hub of a bicycle.
When you mount a lockring you thread the lockring onto the hub counterclockwise. Use a lockring tool to tighten the lockring firmly against the cog. Always make sure the lockring and hub threads are completely engaged prior to use.
So remember a lockring is reverse-threaded, so you will have to turn it to the left to tighten it. Always us a tool to tighten the lockring on the fixedgear hub.

Also check if you have the correct thread. We sell lockrings with for example Italian and British threading. Check with the manufacturer of your hub which thread of lockring you need.
Never use the wrong thread lockring on your fixed gear hub, it might seem to work at first but it will damage the thread. We sell affordable and simple lockrings for your fixedgear from brands like Sturmey Archer and BLB.
There are also higher quality lockrings for your fixedgear from for example White Industries. Just make sure the lockring is put on correct on the thread and use a special tool for the lockring on your trackbike.