The goal of the wonderful game Fallout 2. You are a descendant of the Vault Dweller from the original Fallout living in the small village he founded, but a drought is killing off your brahmin (mutant cows) and starving the children. The village elder summons you and tells you to find the GECK.
The GECK is an item all Vaults were equipped with before the war started; it's rumored to have the power to create a paradise from mere dust and was meant to be used when the inhabitants of the Vault finally left. Your goal is to find this mysterious item somewhere and bring it back before everyone dies. All you know is... well, that's it, really. Be sure to watch the intro movie, though, as it shows you kinda what one looks like.
Stop reading now unless you want to be mildly spoilered. Look at the bottom of this writeup if you want to be even more spoilered.
I mean it!
Of course, as in the original Fallout and its silly water chip (it's fun to run around telling everyone you've gotta find the water chip), this isn't actually the goal of the game. Even if you get the GECK in a week, something I won't mention happens. Which is good, because it encourages you to do all the sidequests.
If you're really desperate to know what the true goal is, go look at GameFAQs. Be warned, this will make the game a lot less fun, as it'll probably also tell you how to get the Super Hardened Impenitrable Cheating Power Armor before anything else.
may this former nodeshell rest in peace.
spoiler: the GECK is in Vault 13. big surprise there, huh?