Leave it to Krugman:
The essence is really quite simple: regulation of insurers, so that they can't cherry-pick only the healthy, and subsidies, so that all Americans can afford insurance.
...[W]hat it means for the individual will be that insurers can’t reject you, and if your income is relatively low, the government will help pay your premiums.
That's it. Any commentator who whines that he just doesn't understand it is basically saying that he doesn’t want to understand it.
The article he's reacting to is also worth reading.