Short version is the health care law passed a couple years ago included a provision referred to as the "individual mandate" which requires everyone to buy health insurance (exceptions made for those who can't afford to, etc etc), the idea being that if you force everyone to buy it then young healthy people who don't normally do so will be added to the insurance companies' groups, lessening the overall risk, which would allow them to lower costs. It was challenged on the basis that the government doesn't have the authority to compel a private citizen to buy a product from another private citizen, but was upheld on the theory that it's a de facto tax and the government does have the authority to tax.
Way over simplified, but there you go.
*Edit* - Correct, there's actually a tax penalty for not buying insurance. So the court basically said that the government can't force you to buy a product, but they can tax you if you don't. It's an extremely technical legal argument, and it still seems like splitting hairs to me, but I no longer think the decision was wrong per se...