Infinity stones are MacGuffins of the highest order

To put it another way, the rocks are MacGuffins of the highest order. A MacGuffin, in film school parlance, is a plot device (tangible or otherwise) that motivates the characters without necessarily explaining why. In Marvel’s world, infinity stones represent an ultra-powerful force that bad guys want and good guys want to make sure the bad guys don’t have, even if they don’t see the bigger picture.

Marvel Studios started its film series relatively grounded, explaining away some of the crazier elements with exaggerated science and technology. Over the last 11 films, it's been slowly and deliberately introducing more cosmic and magical elements into the foreground — all leading up to 2018’s Avengers: Infinity War Part 1, where Thanos (Josh Brolin) will finally jump into the Bad Guy Foreground. It’s a lot to ask an audience to keep track of everything, and for the most part you can enjoy the films without a degree in comic book history. But like Thanos himself, Marvel’s ultimate plan is nothing short of the entire universe (of entertainment). It’s probably best to start learning the fundamentals now.

Infinity what now?

"Before creation itself, there were six singularities, then the universe exploded into existence and the remnants of this system were forged into concentrated ingots... Infinity Stones."

— A blonde-haired Benicio Del Toro in Guardians of the Galaxy

That’s the most succinct explanation we get about the infinity stones in all of Marvel’s first 11 movies. The Big Bang’s leftovers are represented by six colored stones with powers that don’t otherwise exist in the universe. Each has its own name: space, mind, reality, power, soul, and time. We've only seen the first four on screen so far, with soul and time still awaiting their on-screen debut.

I assume there’s a comic book connection somewhere.

Of course! Infinity gems, first known as Soul Gems, originally debuted in 1972's Marvel Premiere #1. There's a lot we could say here, but let's boil it down to the essentials: the Mad Titan Thanos wanted the six soul gems to destroy the world. He was defeated. He was later resurrected, and — surprise, surprise — went back to gathering infinity gems from all across the galaxy.

This time around, which in our meta human world is circa 1990, Thanos amasses all the gems and places them in his infinity gauntlet (read: glove), and destroys half of the population of the universe. He is once again defeated by a merry band of superheroes across a number of various comic book issues.

It's worth noting that in both events, the Marvel character Adam Warlock played a very key role in Thanos' defeat — but seeing as he's not (yet) part of Marvel's Cinematic Universe, we're not going to delve down that path. (Worth noting but wholly speculative: some elements of Warlock, specifically wielding an infinity stone on his head, seem to have been mapped onto the film version of Vision.)