The Second Amendment has not been the subject of much Supreme Court discussion through the years. To the extent it has been discussed, the Court has described the Second Amendment as designed to protect the ability of the states to preserve their own sovereignty against a new and potentially overreaching national government. Based on that understanding, the Court has historically construed the Second Amendment as a collective right connected to the concept of a "well-regulated militia" rather than an individual right to possess guns for private purposes.
In Heller, the Court reinterpreted the Second Amendment as a source of individual rights. Washington D.C.'s gun control law, which bans the private possession of handguns and was widely considered the most restrictive such law in the country, became a victim of that reinterpretation.
The Court was careful to note that the right to bear arms is not absolute and can be subject to reasonable regulation. Yet, by concluding that D.C.'s gun control law was unreasonable and thus invalid, the Court placed a constitutional limit on gun control legislation that had not existed prior to its decision in Heller. It is too early to know how much of a constitutional straitjacket the new rule will create.