But if you want to be the real root of your system by using the "su" command in Terminal then when you're asked for the Root password, then you're stuck at it unless you change the default password (which is chosen using a random hash during the installation process) to a desired one.
So, without using any GUI tools, how can you change the Root's password in Ubuntu or Linux in general. We'll it it's simple actually, first open your Terminal and issue the following command...
sudo passwd root
Now you'll be asked to give a new password (twice), then enter the desired one. But remember, by default the root password is chosen as said before by using a random hash, so, if you override it using a weaker password, then your Linux may be easily hacked. So when choosing a password, make sure to give a strong one!.