sudo su - will elevate any user(sudoer) with
su - anotheruser will switch to user environment of the target user, with target user privileges
sudo su - username mean?
Just repeating both @dr01 and @OneK's answers because they are both missing some fine details:
su - username - Asks the system to start a new login session for the specified user. The system will require the password for the user "username" (even if its the same as the current user).
sudo su - username will do the same, but first ask the system to be elevated to super user mode, after which
su will not ask for "username"'s password because a super user is allowed to change into any other user without knowing their password. That being said,
sudo in itself enforces security by by checking the
/etc/sudoers file to make sure the current user is allowed to gain super user permissions,and possibly verifying the current user's password.
I would also like to comment that to gain a super user login session, please use
sudo -i (or
sudo -s) as
sudo su - is just silly: its asking
sudo to give super user permissions to
su so that
su can start a login shell for the super user - when
sudo can achieve the same result better it by itself.