What is root?
Root is the name of the most powerful account on a Debian installation. The root user account can do everything on the machine. Root is also known as supervisor and administrator. Root's home (~) folder is /root.
At installation time, you are asked whether you want to use the root account or not.
if you want to (the default), you'll be asked to provide a complex password for root. Use a strong one!
if not, no root account is enabled and the password of the first user created will be used for administration tasks.
If you forgot your password, read "I forgot the root password!"