Just slide this on your knife to help keep a steady angle while sharpening your knife on a stone. The white strip is less abrasive than the cheap steel guides you sometimes see so you're less likely to scratch the stone (and the plastic is less likely to scratch the blade face). Note that the angle created will depend on the width of the knife blade... it will give you a nice Japanese angle with a santoku or most chef's knives, but the angle will be higher with a small blade like a paring knife, and very low with a super wide blade like a Chinese chef's knife. But it can still be very handy, especially when you're just starting out and want to concentrate on your technique without also having to worry all the time about maintaining a consistent angle.
Please note: Please test this clip carefully and make sure it and the knife are very clean (no grit contamination!), to avoid potentially scratching the knife. And when putting it on or removing it, it's best to avoid sliding it horizontally, again to guard against perhaps scratching the knife blade. Of course freehand sharpening is always the best, so you might find you never need the "training wheels" of the clip.