Extraterrestrial thing found in my suspension fork!
It's an elastomer damper, see figure 6 on the image below. Without a damper, the fork would bob uncontrollably like a pogo stick, which is not good at all when riding a bike. Elastomer dampers are a sign of low-end forks, while more expensive forks use oil damping or other forms of damping. You can replace the stock piece of elastomer with less rigid or shorter one to adjust the fork feel.