Sam2 – really? Do you know for sure that the ORIGINAL ORIGIN is christian and not Jewish? I’m just curious, because I had thought it was of Jewish origin, but I really have no clue.
In any case, it’s just a mashal, and not even a real mashal. In a real mashal, every word is true and represents something real. For example, in a mashal about a king, the nimshal will tell us that there really is a king, but the King is really Hashem and not a flesh and blood king. In this “mashal”, I don’t think there is an exact parallel/nimshal. I think it is just meant as a nice idea to teach us something, which it succeeds in doing (although perhaps, if it really is of christian origin, it should not be used).
The basic idea is that we create our own Gan Eden or Gehinnom through our own actions.
The idea from Rav Avigdor Miller quoted by iacisrmma above is a more precise definition of Gan Eden, since Gan Eden really is about learning Torah and getting close to Hashem.
It is true in this world as well. You can have Gan Eden in Olam Hazeh if you appreciate Limud Torah and doing Mitzvos. I learned that that is why it is called Olam Haba (the world that IS coming), since we already start creating and experiencing it in THIS world when we learn Torah, do Mitzvos and create a relationship with Hashem. (I think the source might be the Maharal).