It's just a link to Apple's OS X page. You can get it back by dragging a link from Safari down to the Dock but its not really nececcary unless you really want it there again for some reason...
That's one of the first things i delete
It's just a link to the os x webpage (http://www.apple.com/macosx/tiger/ or something similar), which i figure is pretty damn easy to find without wasting dock space.