It's a shortcut that starts the Web browser and takes to the Apple site, if memory serves me. I always remove it; all it does is take up Dock space, since Safari is already in the Dock and has a Apple link in the bookmark bar.
When you control-click on the spring (or just click and wait for a contextual menu), you get a "URL" designation, so I guess it's just a URL to Apple's OS X web page with a custom icon. Unlike everything else in your Dock, the spring doesn't appear to represent an actual file on your computer, otherwise it would offer to "Show in Finder".
You can drag your own Apple URL to the Dock, but I don't think you can get that custom icon. The only time I've ever seen that spring is when I actually installed the OS.