A declaration of victory or superiority. The phrase stems from a 1991 adaptation of Toaplan's "Zero Wing" shoot-'em-up arcade game for the Sega Genesis game console. A brief introduction was added to the opening screen, and it has what many consider to be the worst Japanese-to-English translation in video game history. The introduction shows the bridge of a starship in chaos as a Borg-like figure named CATS materializes and says, "How are you gentlemen!! All your base are belong to us." sic
In 2001, this amusing mistranslation spread virally through the internet, bringing with it a slew of JPEGs and a movie of hacked photographs, each showing a street sign, store front, package label, etc. hacked to read "All your base are belong to us" or one of the other many supremely dopey lines from the game (such as "Somebody set us up the bomb!!!" or "What happen?"). When these phrases are used properly, the overall effect is both screamingly funny and somewhat chilling, reminiscent of the B movie "They Live".
The original has been generalized to "All your X are belong to us", where X is filled in to connote a sinister takeover of some sort. Thus, "When Joe signed up for his new job at Yoyodyne, he had to sign a draconian NDA. It basically said, `All your code are belong to us.'" Has many of the connotations of "Resistance is futile; you will be assimilated"