What does the term 30 mean to a newspaper editor?

In the context of a newspaper editor, "30" is a shorthand way of saying "end of story".

It originates from the old days of typesetting when a piece of lead type with the number 30 was used to mark the end of a story. This made it easier for typesetters to identify where one article ended and the next began.

Even though newspapers are no longer typeset using lead, the term "30" has remained as a traditional way to mark the end of a story.

So, when a newspaper editor says "30", they are essentially saying "that's the end of the article".