1. If you are Lil Wayne: if this case applies the Second Amendment to the states, there's a good chance that next time you're busted on your tour bus it'll just be for the coke and weed! Not your guns! Watch out, America, ridin' dirty could get a little cleaner.
2. If you are NOT Lil Wayne: the outcome of this case may (if SCOTUS decides not to be pussies) tell us if states are allowed to prohibit gun ownership.
So, now that you're interested and not just reading this because your DSL is being super slow and your porn is taking forever to load, a little explanation about what's is up with the Second Amendment. To ensure you're engaged, I'll also insert pictures of some hot chicks with guns. But seriously, it's kind of interesting:
The juiciest of these is the Fourteenth Amendment, which, despite reading pretty simply, has a few clauses in it that have completely changed U.S. law. One such clause reads "nor shall any State deprive any person of life, liberty, or property, without due process of law." This is known as the "Due Process Clause." It may not seem like much, but it has been used by SCOTUS to authorize just about anything they can possibly think of.
But wait, you're saying, I'm no mathematician, but I thought we were talking about the SECOND Amendment not the FOURTEENTH Amendment! Yeah, you're right, chillax, there's more. So, after the Civil War, it started to seem pretty dumb that all these awesome Bill of Rights Amendments only applied to the federal government. SCOTUS finally decided to say that the FOURTEENTH Amendment, via the Due Process clause, could APPLY the other Amendments to the states, i.e., incorporate them.*
But SCOTUS isn't always as smart as we give them credit for, so instead of incorporating all of the Amendments to the states at once, they did it one by one (and sometimes sentence by sentence - the First Amendment alone took at least half a dozen cases to be incorporated). So guess which Amendment hasn't been incorporated? THE SECOND AMENDMENT RIGHT TO KEEP AND BEAR ARMS! And that very issue was argued today before SCOTUS!! And we don't know what's going to happen!!!
SCOTUS has been notoriously lame about addressing the Second Amendment. In 2008 (more than 100 years after the court started "incorporating") they FINALLY heard a case purporting to incorporate the Second Amendment, Heller v. D.C.. But they sucked it up -- while Heller was considered a victory for gun-rights advocates, all it really did was say that the Second Amendment applied to individuals, not just the military (thanks to some super annoying reasoning about "prefatory" and "operative" clauses).
So, the decision in McDonald has the potential to change a whole lot: SCOTUS could finally incorporate the Second Amendment, restricting states' ability to ban guns, or they could deny incorporation once and for all, leaving states free to regulate guns as they please. Either way, there's a huge possibility that McDonald v. Chicago will help the Second Amendment go out with a bang.
*There was originally another option to apply the Amendments to the states, the "Privileges or Immunities Clause" of the Fourteenth Amendment. But The Slaughterhouse Cases put the kibosh on that in 1873. Some people think Slaughterhouse should be overturned and we should reevaluate Privileges or Immunities, but it seems unlikely that SCOTUS will go that route. However loosely it fits, we're probably stuck with Due Process.