It's a seemingly technical question of the order in which a court ought to decide whether there is a violation of a constitutional right and whether a state official ought reasonably to have known that. Saucier required that courts first ask whether there is a rights violation, but it's often more straightforward to say, whether there was a rights violation or not, the state official couldn't reasonably have known it.
Answering the latter question resolves the legal dispute — the rights claimant loses — but it deprives us of a statement about what the law is. On the other hand, if you say what the law is, and it doesn't affect the outcome of the lawsuit — because you've said there's a right but the rights claimant still loses — it violates the principle of avoiding unnecessary interpretations of constitutional law.
Justice Alito writes for the Court:
[Saucier] departs from the general rule of constitutional avoidance and runs counter to the “older, wiser judicial counsel ‘not to pass on questions of constitutionality . . . unless such adjudication is unavoidable.’” Scott, 550 U. S., at 388 (BREYER, J., concurring) (quoting Spector Motor Service, Inc. v. McLaughlin, 323 U. S. 101, 105 (1944)); see Ashwander v. TVA, 297 U. S. 288, 347 (1936) (Brandeis, J., concurring) (“The Court will not pass upon a constitutional question although properly presented by the record, if there is also present some other ground upon which the case may be disposed of”).One might have thought that would lead to the decision to reject the Saucier ordering of the questions and put the question of the scope of the right second in all cases. But this is one of those pro-flexibility decisions. Let the trial judge, in his or her wisdom, put the questions in the order that works best. And not one Justice felt moved to concur to say that a more strictly principled rule should be imposed.