Yep, what Dave said above (hiya dave
)
It's for spring loading the top cup of the headset.
Your top cone should have wee teeth in it, kinda like the inside of a cassette hub or freewheel. You pop it in and pretension it - stops the headset unwinding and getting loose.
I've one here - they're pretty good - after you get them set up right. that bit's kind of a pain...
Brian