It's a laterally mounted shock absorber connecting the steering head to the frame. It prevents the dreaded "tank slapper", caused by steep fork angles, low trail, or poor technique. Most street bikes have them from the factory if they need them. Your Kat should have mild enough steering geometry that you won't need it unless you do a lot of stunting. If so, consider installing a stupidity damper.
Here's a photo.
Here's a photo.
