From wikipedia
The Linux kernel is the operating system kernel used by the Linux
family of Unix-like operating systems. It is a prominent example of
free and open source software. The Linux kernel is released under the
GNU General Public License version 2, and is developed by contributors
worldwide
It is the heart of linux and any linux distribution is depend on it
while ubuntu is being updated every day. The linux kernel is also updated. So you have a couple of old archived kernels in your boot partition and you can safely remove them using this command:
sudo apt-get purge $(dpkg -l linux-{image,headers}-"[0-9]*" | awk '/ii/{print $2}' | grep -ve "$(uname -r | sed -r 's/-[a-z]+//')")