At its most simple level, its a rebellion against a cruel and uncaring dictator in a fantasy world.
The magic is based on metals. Allomancy is the the ability to ingest certain metals, "burn it" inside of you, and get a supernatural ability. Most people can't burn metal. Some can burn one metal and get one power. A very few can burn all the metals and are called "mistborn".
Each metal does only one thing, and its alloy does the opposite. Iron will let you push a metal object for example, while steel will let you pull it. So, someone who burns steel could hunker down behind a wooden shield and draw all the enemy's arrows, and someone who burns iron could push them back at them, or "shoot" coins. Physics still applies, so if the object is heavier than you or can't move, you will either be pulled toward it or thrown. So, iron would let you "fly" up a wall if you pulled on a fixed piece of metal on the gutter. Steel would let you "fly" if you pushed off a coin on the ground, but only in straight lines. That's why mistborn are so powerful, they can combine the powers to do things. If you just pushed off a coin and landed hard, you'd hurt or kill yourself. Mistborns can burn pewter to make themselves tougher to counter act that. Someone who only burns pewter can make themselves tough and strong, but has no way to counteract gravity or intercept metal projectiles.
The nobility have almost all of the allomantic power, but a few halfbreeds in the peasant population also has it, and they lead a rebellion.
DON"T RESEARCH IT, JUST READ IT. ITS AWESOME.