What is "root" on Linux?
On Linux, root is the username of a special user created to perform administrative tasks on the operating system. When you install Linux Mint, or any typical Linux distribution, two users are created: one user is your own user, with a username you specify, and the other user is root. It's always called root, not admin or anything like that. It's just root.
Note: the term "root" may also refer to root directory, which on Linux is located by the filepath /.
On Linux, a normal user can't read or modify files and folders owned by other users. The exception being the root user, who can do that. This means that your user can't touch files owned by root. By default, every file on the system is owned by root. The only exception being files in your user's home directory. This means that you can't touch anything except the files you created yourself, and some default files automatically created in your home directory.
The root user also has the ability change the ownership of files, transferring them from one user to another. In particular, if you run a terminal command as root with sudo, any files created by that program will be owned by root. You can transfer them back to yourself with the chown (change ownership) command.
A superuser (a user whose username or group is contained in the /etc/sudoers file) is a user that can use the sudo command to execute terminal commands as root. In other words, a superuser is the equivalent of a user with administrator-level privileges, but the commands executed with these privileges are, in the system, executed as root, and not as the user who is superuser. Being an administrator on Linux just means you have the power to tell root to do something for you.