Those are old kernel boot files - when you install/update to a new kernel the old files arent removed. In your boot menu you will see all the kernels listed with latest being default. That way you can, if any unforeseen issues occur with newer kernel, boot to previous one.
I think you'd be better off using the package manager to remove old/unused kernels rather than delete files individually. You don't want to be removing resuce kernels etc.
You'd want to be checking your running version first, to ensure you don't remove that kernel. (I know yum prohibits removal of current running kernel not sure about dpkg!)
make a note of the ouput here
dpkg --list | grep linux-image
make a list of previous images EXCLUDING the running one
Remove old kernels:
apt-get purge linux-image-x.x.x.x-generic
Its safe to remove anything older that your running kernel
You may see a kernel something like "linux-image-generic-lts-quantal" thats your failsafe - dont think you want to remove that.