To make what the other answers already stated perfectly clear: That other user is not only "root as much as you" (Videonauth's answer), they can also become you (switch to your user account).
This is because with superuser privileges, one can switch to any account.
You probably know
which is one option of opening a root shell if root does not have a password set (so you can't just log in as root directly).
su is short for "switch user". What user does it switch to? None is stated, right? But from the man page, we can learn that:
Invoked without a username, su defaults to becoming the superuser.
So this effectively is
sudo su root
if you didn't rename
root to something else.
If you just run
su <someuser>, you will be prompted for a password. So if you run
su root, you're prompted for root's password (which doesn't exist in Ubuntu by default, so you can't log in (note that no password being set means there is no means of logging in via a password which is different from the password being the empty string)). But if you run
sudo su root, you're prompted for your own password. And you're only prompted for it by
sudo received your password, it runs the command it received as parameters with superuser privileges. As one is able to switch to any account when having superuser privileges, a password prompt is not necessary.
So by executing
sudo su <yourusername>
, the other sudoer can log in as you.