What does root@localhost stand for?

Well, normally root is your computer administrator, and localhost is your computer right in front of you.The at mark is the same as the at mark in the email address.This organization (computer) is on the right side of the at mark and the username is on the left side of the at mark.

Because using a computer with administrator privileges can destroy the operating system/computer itself in one misoperation, it is recommended that you use an account that does not have administrator privileges during normal use, but often requires administrator privileges when installing or changing settings.

root@localhost would not be so wrong if you thought it meant that you were using the computer in front of you with administrative privileges.