The On-One Headset Lock is a replacement for the traditional star-nut that fits in the top of your fork steerer tube.
It basically consists of a headset top cap, a plug that sits in the bottom of your steerer tube, and a long bolt that goes through the whole lot to tighten the assembly up.
Unlike a star-nut, it is reusable should you decide to change your forks - simply loosen the bolt and the plug will drop out of the bottom of the steerer. This method of headset-tightening is popular with the dirt jumping fraternity, thanks to simplicity and super strength. It's a big, solid bolt with nothing to slip.
Ensure you check your forks for compatibility - if you can't access the bottom of your steerer tube from underneath the fork crown you won't be able to use this. Nor if you need a steerer longer than 250mm.