What does a 5-4 decision in a case like Boumediene v. Bush mean?

In Republicanland, it means everything the Supreme Court has told them they've been wrong about for the past 7 years is one vote away from being suddenly right. And staying that way forever.

One more vote, and torture isn't unconstitutional.

One more vote, and indefinite detention on the president's say-so isn't unconstitutional.

One more vote, and warrantless wiretapping isn't unconstitutional.

And do we really need to follow the implications in other arenas? Surely no executive branch official could be forced to testify at Congressional oversight hearings. And don't even ask about reproductive rights. John McCain (for whom at least a very few a very few Hillary Clinton holdouts insist they will vote in November, presumably as punishment for Democrats who failed to grasp their wisdom) could not be more overt about his desire to eliminate them:

John McCain believes Roe v. Wade is a flawed decision that must be overturned, and as president he will nominate judges who understand that courts should not be in the business of legislating from the bench.

One vote away.

One more vote, and everything you thought you understood innately about the nature of our system of government can change, and the insane right-wing trolls who've astounded you with their seemingly otherworldly interpretations of constitutional law will be right, and will claim they've always been right, but for the interference of "liberal, activist judges."

And they will laugh in your face.

But in the meantime, they'll be spitting in it. Like Sen. Lindsey Graham does, when he says he's so sure the Court is wrong in Boumediene that he's willing to amend the Constitution to prove it:

Sen. Lindsey Graham (R-S.C.) vowed Thursday to do everything in his power to overturn the Supreme Court’s decision on Guantanamo Bay detainees, saying that, "if necessary," he would push for a constitutional amendment to modify the decision.

These are the Republicans with whom -- if we're to believe the buzzwords -- we're supposedly obligated to seek "bipartisan compromise."

There's no such thing as "compromise" with people who believe that you and your "correct" understanding of the Constitution, what it says, and what it means is more properly viewed as being merely "pre-wrong."