Take a gander at the footnote above from the Chief Justice's opinion. He's discussing what might happen to someone who defies the individual mandate to buy health insurance. That person would be subject to a penalty, which Chief Justice Roberts--historically--determined is a tax. (You might have read 5 billion articles about that this week, but here's another one.)
But what happens if you don't pay the tax? As the Chief Justice notes in parentheses, you can't actually be prosecuted for that. In fact, the Affordable Care Act's mandate is even less enforceable than that; here's the section that Chief Justice Roberts cites:
Read section (2). It says that, notwithstanding any other provision of law, if you don't buy health insurance as required by the individual mandate and you also fail to pay the "penalty" (all as set forth in the rest of 26 USC § 5000A), then the government can't prosecute you criminally or enforce any other criminal penalty. Moreover, the IRS can't go after any of your property the way it usually does (through tax liens or levies). In other words, the individual mandate is pretty darn toothless.
That does not mean that the individual mandate is optional. There's a "penalty" (tax!) due to the government if you don't comply with the individual mandate, and it's still against the law not to pay the tax. The IRS could presumably sue you for the tax in civil court, or perhaps take it out of your tax refunds in future years (or maybe even gin up some other enforcement mechanism that isn't forbidden above). In addition, if you don't paid a tax that is due, you cannot truthfully say that you have paid all your taxes, or that you are in compliance with all applicable law, which could make it difficult to get a loan or get into law school or get a government job--to name a few examples. However, for so long as that provision above stays in effect, nobody is going to be hauled off to Fort Leavenworth or have their boat taken away because they refuse to buy health insurance and also refuse to pay the tax that's due as a result.
Perhaps, now, everybody can chill.