An extremly common videogame concept
*. A fetch quest is simply that: A quest
something. Go to foo
and retreive bar
. It matters not what bar is, or where foo is located. When someone gets something in return for something else, that is a fetch quest.
However, the phase is commonly used when the Something-to-be-retrieved is some sort of powerful and/or magical artifact**, and the place to retrieve it from is either a deathtrap itself, or the journey to it is arduous and trying.
* Although fetch quests are by no means limited only to videogames.
** Runner-up for the most commonly fetched item? A princess or other sort of hostage. Only you can Rescue the Princess.