What does "there and back again" mean?

Some may have heard (or read) of the alternative title of The Hobbit by J.R.R. Tolkien, which is There And Back Again. I figured it might literally mean something like I went there and came back here again, and figuratively referring to some trip that is significant in some way, where I refers to the protagonist of the story.

I am not too sure if my interpretation is correct, so I hope I can get some confirmation here.

Also, I have one side question: I know it's probably not an idiom, but is it alright to use it as if it were an idiom? I think its meaning is pretty apparent, and it's probably just my lack of confidence that's making me ask this question. For example, if I went to stay in Australia for at least a few years, but then circumstances forced me to go back to my country after a month, so the time I spent there felt almost like an adventure, can I say there and back again? Note that I am trying to emphasize the "adventure" part of the round-trip.

It's referring to the path of a journey, eg

"So, where did your adventure take you Bilbo?"
"There and back again"

I wouldn't say that it's an idiom, although it depends on what you use it for I suppose. An idiom's meaning isn't obvious, so if you used to to refer to something that only you understood the meaning of then you would be using it as a sort of idiom. It's not, as far as I know, a commonly used idiom, by which I mean that if people say it then they mean what it seems like it would mean.

EDIT: to expand on the whole idiom thing. Here's a little flowchart, asking about any phrase.

• is it literally true?
  • YES: it's simply a description.
• NO: is it obvious what it means, without prior exposure to the phrase?
  • YES: it's a figure of speech
  • NO: it's an idiom.

Bearing that in mind, if, for example, you wrote a blog post about your year in Australia, and called it "There and back again" then we can apply the above test. It's going to answer "YES" to the first question, since you have been literally been there (somewhere) and back again.

So, it's just a description, albeit one that is a bit vague. It also contains a reference to The Hobbit. People who get the reference might think that it was an adventure, possibly with some difficulties. Containing a reference to something doesn't make it an idiom, it's just a sort of bonus feature.