It means end of story. i.e. it's what you typed (on the typewriter) to indicate there weren't any more pages which isn't necessarily obvious with strict inverted pyramid writing. No one seems too clear on the origins AFAIK although a leading theory is that it came from a telegraph standard.

It's three exes (XXX) written as Arabic numerals (though they were never Roman numerals to begin with). "XXX" is an easily-identified marker; "30" is just cargo cult stuff.

That's one of a variety of explanations but no one seems sure of the actual answer, e.g. [1] In any case, it was widely used for a long time.

[1] http://archive.saila.com/journalism/thirty/

Non-mobline link:

https://en.wikipedia.org/wiki/%E2%80%9330%E2%80%93