If you’ve been using Ubuntu Linux for a while, then you probably have a number of Linux kernel updates that have been installed on your system. (I’ve got a total of 3 kernels in my Karmic system already.) More here
In all likelihood, these updates get installed and you boot into the latest kernel, never to use the older kernels again. But these old kernels are still hanging around on your system, cluttering up your grub boot screen and taking up space on your hard drive.