I'd like to explain that to you, but I've got to get my notes together for my federal jurisdiction class, where we happen to be studying the very aspect of habeas corpus law that you need to know to understand what Roberts is talking about.
From the Roberts opinion:
Under the Antiterrorism and Effective Death Penalty Act of 1996 (AEDPA), however, a state-court decision can be set aside on federal habeas review only if it is "contrary to, or involved an unreasonable application of, clearly established Federal law, as determined by the Supreme Court of the United States." 28 U. S. C. § 2254(d)(1). When this Court considers similar challenges to the same jury instructions five separate times, it usually is not because the applicable legal rules are "clearly established." The Court today nonetheless picks from the five precedents the one that ruled in favor of the defendant -- Penry I -- and anoints that case as the one embodying "clearly established Federal law." In doing so the Court fails to give any meaningful weight to the two pertinent precedents subsequent to Penry -- Graham and Johnson -- even though those cases adopted a more limited view of Penry I than the Court embraces today.