Ah, the Tesseract, that funky glowing blue box that Nick Fury is hoping to turn into a power source for the whole world.

But it also ended up becoming a MacGuffin…and it ended up powering The Avengers instead.

Although it sounds either like a) a grade-school insult or b) a delicious processed breakfast sandwich, a MacGuffin is actually a plot device. The word is attributed to Alfred Hitchcock, who explained that it doesn't really matter what a MacGuffin is—it only matters that the characters want it. (Source)

MacGuffins in the movies can be anything from fancy black birds to plutonium hidden in wine bottles to whatever the heck's in that briefcase in Pulp Fiction. Here, it's the Tesseract, a doodad from Asgard that the Nazis were playing with in the first Captain America movie, and which apparently holds infinite power to do whatever the owner wants. (Stay tuned for future Marvel movies for more on that one, true believers.)

That makes it a nice stand-in for the nature of power itself. In Nick Fury's dreams, it's something that can bring the world together, or as Loki puts it:

LOKI: To have the Tesseract, to have power, unlimited power. And for what? A warm light for all mankind to share.

Of course, Loki has entirely different plans for it, and we're pretty sure that they don't involve summer picnics and kids holding hands. It's all about whose finger is on the button…which is why it's telling that Fury lets the Tesseract go with Thor at the end: convinced that Asgard can use it more wisely than us squabbling chimps down here on Terra Firma.