In Hollingsworth v. Perry, the Supreme Court essentially declined to rule on Proposition 8, the gay-marriage ban in California. That leaves an earlier district court ruling overturning the ban in place.
More specifically, the Court ruled 5-4 that those who appealed a decision throwing out Prop. 8 did not have legal standing to proceed.
Here's SCOTUSBlog's Amy Howe's plain-English description* of the background of the case:
After the two same-sex couples filed their challenge to Proposition 8 in federal court in California, the California government officials who would normally have defended the law in court, declined to do so. So the proponents of Proposition 8 stepped in to defend the law, and the California Supreme Court (in response to a request by the lower court) ruled that they could do so under state law.
But today the Supreme Court held that the proponents do not have the legal right to defend the law in court. As a result, it held, the decision by the U.S. Court of Appeals for the Ninth Circuit, the intermediate appellate court, has no legal force, and it sent the case back to that court with instructions for it to dismiss the case.
“We have never before upheld the standing of a private party to defend the constitutionality of a state statute when state officials have chosen not to. We decline to do so for the first time here,” John Roberts wrote in his majority opinion. He was joined by Antonin Scalia, Ruth Bader Ginsburg, Stephen Breyer, and Elena Kagan.
*Update: My initial explanation of the back story here was wrong, so I've just turned it over to Amy Howe.