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The answer was on this site or on Ask Ubuntu.SE. It explained use of su, sudo and what root account means. It had lots of pictures, one of which is this one:
How do I go about searching for the answer?
Google it? You can use the Stack Exchange network search as well, although that's just a frontend for Google. The per-site search doesn't support looking up an image by URL
It was How to NOT become a root user? Are administrators root?, on AU